The compression numbers are fine. Sorry if I’m not a whole lot of help at this point but I will continue to think on it a bit.
Never worked on one of these things but have had hands on a few of those D50s. That was a long, long time ago and I do not remember much about them at all.
They must not have been bad little trucks because I remember the worst POS plainly… 
hi everyone! thanks for all the suggestions and comments! really helpful, and i have some updates.
spoiler: i’ve tuned it so that it’s got 80% of its power back, with only quite rare little backfires! here’s how we got there–
i tested the vacuum with a mityvac i have around, and the results were interesting. the main “symptom” is that the needle flutters very quickly back and forth about 1 inch Hg. this could be that the gauge doesn’t have a damper, apparently, but also might be useful info.
otherwise, at idle and warm the vac is around 17-18 inches. not great, but not bad afaik. rev quickly and release, and it drops to zero then back up quickly to around 23, then settles back. so – no clogged cat. maybe worn rings. rev high (no tach, but in vicinity of 3k rpm by ear), and it goes down on accel, stays level around 18 or so when even revs, then goes up a bit when decel. seems normal. otherwise, no big issues (eg. spontaneous drops in vac, etc.)
so. if the flutter isn’t an artifact, then i think it could mean one spark plug is underperforming, or a valve is a bit tight or got friction or something.
so i start to explore the sparkplugs. bingo - the #1 plug is more carbonized that the others. testing the gap, it’s also a lot smaller – like .030 when the others are .039, which is spec for that plug. (maybe the #1 got bumped a lot as i was pulling it over and over to get TDC correct.)
however – spec for the vehicle is .044! i check, and the original plugs before i put in new ones are also at .039, so it’s been running with subpar spark the whole time.
gapped the sparks, and there’s a distinct improvement in performance. can accelerate up hills, much less bucking and hesitation on accel, and much fewer backfires (which only happen at low rpms under load).
so this is exciting!
after gapping the sparks, the flutter in vacuum is less but still present. when i rev high, it sometimes goes away completely, but then back at idle. unlike previously, it seems as if it’s mostly at the high point, then once per cycle (ie. per crank or cam rotation) it drops about 1 inch. this makes me think maybe one of the plugs is still acting up, or maybe a valve has problems.
anyone have suggestions for how to further explore this?
at this point, with how well it’s now running at correct 7 deg BTDC timing, i’m thinking the timing is no longer a likely cause of remaining problems.
(in related news: on the suggestion above, i thought to clean the MAF. pulled it and sprayed with CRC MAF cleaner, waited for it to dry, then put it back in and it seemed to make things better! wow! THEN i read the fine print that said not to use the MAF cleaner on karman vortex MAFs like in the mitsubishi – it can destroy them! argh! but things seem fine so far, knock on wood. lucky i have a spare from the other vehicle.)
again, thanks for everyone’s input! appreciate further thoughts!
absolutely! i’ve never experienced ping/detonation, that i’m aware. it’s been all backfires, mostly little ones early in the exhaust. my understanding is that’s an “incomplete combustion” problem more than a “running lean” one.
A fluttering vacuum gauge indicates a mechanical issue, probably a valve too tight. I would check valve clearance and adjust as needed.
I think there is a problem going on and it is not the gauge.
The flutter could point to an air leak or miss.
I’ll throw something out there that is very rare but I have run across it a few times. That is a weak valve spring. The springs were not broken; they just had a lack of spring tension.
With the valve cover off I went down the row pushing the valve collars with my thumb. Eventually I ran across one that depressed easily. Aired the cylinder up, changed the spring out, and all was well.
The auto machine shop who had just serviced the cylinder heads obviously screwed the pooch… 
I am not saying this is the problem here. Only a possibility but I’m having a hard time seeing something like a weak spring causing a major loss of power unless multiple springs are involved.
It just seems to me that throwing in a lot of timing advance to gain power means you’re trying to override an existing problem. Will continue to ponder this…
When you write of backfires, do you mean it’s blowing back through the intake or that it’s exploding fuel in the exhaust?
Because I’m thinking you have some sort of valve problem, and this has hydraulic valve lash adjusters which do sometimes fail. There are a few videos on YouTube and maybe more info on specific Mitsubishi forums.
Congrats, sounds like you’ve made good progress. I think given the current state of affairs, the next steps are
Vehicles of this vintage (early 90’s) often use an air-bleed screw that allows a small amount of air to bypass the throttle valve to set idle rpm. If you engine works that way, this setting is critically important to engine tuning. The engine computer assumes this has been done correctly as part of its air/fuel ratio and ignition timing calculations. Make sure you have the instrumentation necessary to get the idle rpm exactly correct. I use an oscilloscope for this.
The vacuum readings you are getting sound normal.
To find TDC accurately, you do NOT stick the chopstick in the #1 cylinder. You stick it into the #2 and #3 cylinders. When the chopstick drops to the exact same depth, then the engine is at TDC.
Edit: My bad, before everyone jumps in to correct me, I was thinking of a V8 or 6 cylinder. That does not work on a 4 cylinder. You do have to use the chopstick in the #1 cylinder, but 90 degrees before TDC, make a mark on the stick, then go 90 degrees after TDC to see if you are back at the same mark. If so, then the harmonic balance is accurate.
With the engine at TDC, if the timing marks on the harmonic balancer line up, then check for cam timing. You’ll need the service manual for this but the mark on the crank timing gear will be lined up if the timing marks line up, you only need to check the cam timing gear.
If that lines up, then you need to make a mark on the side of the distributer body where the #1 spark plug tower is on the cap, then remove the cap and see if the rotor points to that mark. If not, the distributor has to come out and be reinserted correctly.
Once all that is done, then the ignitor or cam position sensor can be checked and adjusted for alignment. BTW, if you have a cam position sensor instead of an ignitor, then the timing is set by the computer and not the distributor so there won’t be any mechanical or vacuum advance. But for the computer to get it right, the distributor and cam must be aligned correctly, otherwise GIGO. (garbage in, garbage out).
My post was based on past experience George. And I have dealt with that situation on several automobiles of various makes. And of course here in Mississippi the only herring we have are smoked and in a can and not very appetizing.