Help w/ old 1960 t-bird w/352 running issues...

I have a 1960 ford T-bird- AKA the rubber ducky…

For some reason, I cant seem to get it to run well at all- it has a 352 engine, all orig, 52 k orig miles. it starts, and idles damn near perfect., but has NO pickup, like the carb isn’t giving enough fuel, or the timing is off. I’m a good mechanic, and have rebuilt the carb to spec, set the timing to spec, the distributor is advancing, has new points, plugs, wires, cap, rotor, etc. the engine has good compression, but I can barely get the thing above 40 mph, I was wondering what the next thing I would look for? the secondaries on the 4 bbl carb aren’t opening, my next guess is a massive vacuum leak, maybe in the manifold? I’m really puzzled, and hate to pull the manifold, as it weighs like 80 lbs, and is hard to do by myself… any other suggestions? I’ve checked everything I can think of other than that…

Percy.

put a vacuum gauge on it check your idle vacuum and vacuum under load

check the fuel pressure

Secondaries not opening is a clue to a vacuum problem but it should go more than 40mph without them.