It sounds like your carb float needs to be adjusted, but it could also be your coil wire is shorting out on the block or is breaking down internally. You can buy just a new coil wire and they are cheap so you might try that. Testing the wire with a continuity tester will not tell you if it is breaking down under a load.

Pull the plug wires from the top of the distributor cap and look for a blue powder on the wire plugs on down in the plug holes. The blue powder indicates the presence of arcing, building up the resistance level so your spark is not strong.

It could also be a leaking intake manifold, but this is not common on this engine.

Have you checked your ignition timing? If this engine has a set of points have you checked the dwell?

Also check your vacuum advance.

I know this is a lot of stuff to look at, but all of it, except the intake manifold, is cheap and easy.