If I get everything back up and going without issue Im going to keep her off the highway for a while until I figure it out.
Iād think youād have no problem at 65 mph with everything running right. Trouble with the distributor (centrifugal/vacuum advance, bad bearings) or the carb/vacuum leak could be the problem.
I forget, have you checked the compression?
I havent checked the compression. Im sure the engines worn as it ate a quart or so of oil on that hour trip. I mean I hit 65 fine, but the rpmās were much higher then a modern day car. I felt like I was working the engine too hard. I bet I was at atleast 3000 rpm and this thing idles below 500
Odd, rpms should be fairly low on an old low-rpm motor. So maybe the tranny wasnāt in the right gear, or itās tired and you had use a lot of gas, making it downshift.
I think that 3000 RPM at 65 mph would be fairly typical.
My '65 GTO did 3000 RPM at 60 mph, and that was with a 3.23 rear-end gear. 3000 RPM is still the low end of mid-range, and Iām pretty confident that your Caddy motor could do that all day long without breaking a sweat.
A stock '65 GTO with 205/75-14 tires and a 3.23 rear-end going 60 mph would be turning 2500 rpm with a 1:1 high gear.
I checked a classic car data base and the compression ratio on that motor is 7.5 to one, the rear end ratio is 3.77 and vehicle weight of 3880 lbs. I have a friend who has a 36 Buick with the 92hp straight 8 and has a 4.44 rear end. At 65 -70 sheās screaming. You could take it down to 15 mph in 3rd and still would accelerate without lugging. The URL for the website is http://www.classiccardatabase.com
Only problem with that website is its referring to the 3 speed manual with the 3.77 rear end. Mine is the 4 speed hydramatic which I didnt see listed except for the series 62 model. That has a 3.36 rear end, which Im assuming mine has.
Check for vacuum leaks. You can start it up and spray Gumout around vacuum fittings/connections and carb base mounts and a leak will suck in Gumout and it will stall or idle rough. Old engine means anything and everything could be worn. Carb rebuild or replace for sure. Distributor advance weights shot and even point cam lobe can be worn down or maybe weak coil. Those will cause slow speed rough running. When engine is cold does it start right up (be sure choke is functioning)? If not then motor might be worn out everywhere.
"A stock '65 GTO with 205/75-14 tires and a 3.23 rear-end going 60 mph would be turning 2500 rpm with a 1:1 high gear. "
Iām sure youāre right.
Trouble is, Iām sure my tachometer was right too. It was quite linear, showing 1000 rpm at 20 MPH, 2000 rpm at 40 mph, 3000 rpm at 60 mph and 4000 rpm at 80 mph., all in 4th gear.
It had a later M22 transmission and I know it had a 3.23 rear gear in it because I put it in.
{edit} Let me correct that.
I know it had a 3.23 rear end gear because I put it BACK in after replacing the open differential with a carrier with a limited slip, using the original ring and pinion.
Looks like Xmas got here a bit early for you due to the transmission not being the problem.
Excellent news!
Regarding horsepower, in most cases those figures are optimistic and the actual numbers at the rear wheels where it counts may be very disappointing to car owners if they knew the real ones.
No fuzzy dice for the rear view yet? 
Agree with ok4450 on the hp valuesā¦go on that website I posted and compare 2 identical cars from 1971 and 1972 and look at the hp difference from gross to net. In 72 they changed the way they rated hpā¦A 71 olds with the 350 had 240 hp, in 1972 went down to 160 hp. About a 33% lossā¦Same with the caddy 472, 71 was rated at 335 hp, and in 72 around 220hp. They did not change much with the engine designs in one year. So a 80 hp loss for the olds and 115 hp loss for the caddyā¦Does not make sense. Still comes out to 33 % loss.
@Fender1325ā As I remember the 1953 Cadillac had 210 HP, the Lincoln had 205, and the Chrysler New Yorker had 180. I donāt think horsepower ratings meant .much back then. It probably depended on the horse used as a standard.
So I ran it today just to hear the engine. Sheās not firing on all 8. I pulled some plugs and they looked ok. A few were a tiny tiny bit wet on the threads. I also noticed the intake manifold had a hairline crack in it under one of the studs/nuts. It mustve got really hot. Sighā¦
A vacuum leak due to the manifold crack would cause misfiring and stumbling at idle and low throttle.
Try spraying ether or carb cleaner at the crack and see if idle changes.
They should consider using the stubborn but sure-footed mule but some might confuse MP ratings for āmileageā and not āmuleā. 4x4s with Mule Power ratings would definitely aid in knowing how well the vehicle could climb!
From 2014?? I wondered about Fender and his blue Caddy. And Wesw too. Too bad this has been dredged up from three years ago.
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I enjoyed it a lot.
I did too, but I remember the early 50s well and that Caddy would do an honest 95 to 100 when new and high rpms at high speed were the order of the day. these cars were geared to maximize performance and didnāt have the overdrive transmissions of today. Some of the horsepower ratings were pure advertising fiction, increasing the rating of an unchanged engine from on year to the next by taking the reading at a higher rpm.