The stock setup is a very well developed package. If you modify any one of the three part package (carburetor, camshaft, and distributor) you change a vital characteristic of the engine (vacuum signal and/or ignition advance curve).

Many things can cause your engine to run well at idle but badly at higher rpm. Diagnostics over the internet is a dicey proposition, but we should at least be able to point you in the right direction. One thing to carefully avoid is assuming you have one specific problem and chasing that one idea.

I would start by checking your vacuum advance as described above. I would make sure your vacuum line is connected to the correct port in the middle of a correct carburetor. If you connect an 8BA distributor to manifold vacuum, you get an advance curve backward of what you need.

I would verify that all cylinders are actually firing properly. An engine can sound good at idle when there is no load but not run worth a darn when you try to drive it. Carefully, one at a time, pull each spark plug wire off the spark plug and see how far a gap the spark will jump when the engine is running (probably a very bad idea if you have a pacemaker). The spark should be blue / white and jump at least an 1/8 inch gap (1/4 to 1/2 inch gap is better but not mandatory). A weak orange / red spark means you have problems with your coil, condenser, points, point gap, cap, rotor, wiring going to or in your distributor, or some combination of the above.

If your spark is good, then fuel is probably the problem. I always used to start with spark, but these days with the alcohol laden fuel I equally suspect fuel. One thing to be careful of is setting the float height. You cannot follow the manufacturer's (or rebuilt kit) directions anymore. Today's gasoline doesn't "float" the float as well as the old leaded gas did. You have to set the float much lower than before. I start at 1/4 inch down from the instructions and lower more if required.

Hopefully the above will provide you a place to start diagnostics.