Thanks for the interesting comments, here’s some of my thoughts on this …
My truck runs and drives nearly like new (engine-wise), no complaints. Even on a cold start all I hear is “rr-pop-it’s running”… Doesn’t even get to the second rr. On cold starts it takes a little jigging of the accel pedal the first 15-20 seconds or so to get it to idle well, but after that it idles perfectly. Warm starts, it’s “r-pop-run”. Not even the first full “rr”. It’s main drivability problem is a little hesitation during accelerations. It’s always done that, not a recent thing. Modifying the accel pump setting doesn’t help. HP wise overall, it has plenty for me. But you don’t need a whole lot of HP in a truck. Too much HP in a truck seems dangerous b/c truck’s being top heavy. But a little more oomph during accelerations might be ok.
A chart of HP vs model year for the same engine would demonstrate the HP issue. The 302 engine HP spec fell like a rock during the early 70’s. I don’t have that chart in front of me, but as I recall the first year the 302 was produced was 1968, and that year had 225-250 HP. 250 required a 4B carb I think.
My truck’s engine is rated a mere 140 HP. Most of that fall was likely due to the reduction in compression from 9.5 (1968) to 8.2, but some is probably due to the set up. Like I say, HP isn’t much of a motivation for me, but a reduction in hesitation is. And improved HP might do that possibly. Another motivation is the 70’s 302’s tendency for overheating at idle. The lean-ness of the idle mixture combined with the low advance spec at idle I guess is part of the reason. It’s possible to buy higher volume water pumps to address this, a common thing done apparently for rock crawling with a 302 4x4 truck. To address this overheating at idle problem at the factory, Ford added another vacuum port on the distributor which advanced the ignition a little when the coolant got too hot. But why not just start out with it advanced to begin with? Why make it so complicated? I mean for a seldom driven truck.
I don’t really understand the motivation for a direct intake manifold source for the vacuum advance myself. Like you say, it seems like you want the advance to happen as you open the throttle, which a ported source does. If the intake manifold is the source, when you open the throttle you’d get less advance at throttle openings. But maybe there’s more to it. From what I can tell the early 302’s (like in 1968) used the manifold as the vacuum advance source, right from the factory. Ford then switched to a ported source in the 1970’s for emissions purposes.
The articles I’ve read about this topic (mostly in hot rod type magazines) seem to be pretty adamant that a ported vacuum advance doesn’t produce as good of results in a 302 as a direct manifold advance. My guess is that is may eliminate some of the hesitation during rapid accelerations. Why that would work, I don’t know. @missileman 's post above seems to indicate using the manifold as the source is a common thing '70’s 302 owner’s do though. So they must have a reason. But maybe the reason only shows up on drag strip use? Don’t know. I’m not doing any drag strip racing anytime soon… :