Fred,
This is just a thought, trying to always look for the simple solutions. You said the car sat for twenty years. I assume you’ve got a 2BBL Autolite carburetor. Although you rebuilt the carb, if you didn’t clean the gas tanks and replace the fuel lines, there probably was (and is) a lot of corruption coming from there to the carb. Even with a new fuel filter, some small particles can get through to the carb’s float bowl and partially block the main jets in the bottom of the bowl. This means the idle circuit can be open, therefore good idle. The accelerator pump and circuit can be open, so you have good acceleration when you punch it or jazz the throttle. When your just moving along at a constant mid RPM the engine’s being starved. The check for this is simple; leave the carb in place on the manifold, remove the upper cover from the carb, look down at the jets and see if you can see anything blocking the holes in them. Even if you can’t see anything, take a screwdriver and unscrew the jets, remove them and hold them up to the light to see if the hole’s blocked, If so, take a wire, clear the opening, check the bottom of the bowl for more “crap” and clean it. Reinstall the jets, close the carb up, fire it up and take it for a test run. It takes about as long to do this as it took me to type this. If it sounds like I may have experienced something like this, I did. If you try this, let me know how you make out. Keep in mind, if this is the problem, you’ll need to address the fuel tanks and lines (not as simple).