I've had distributor advance plates short out and cause this problem.
Are you running points or Hei distributor?
If points, set them to .017 and make sure you have a ballast resistor to drop the coil voltage to around 8 volts.
I started with, set the ac/dc arch welder machine to dc, touch off and go then, went to Miller 250 Syncrowave tig machine with high freak start with pedal to now I use a Miller Dynasty 200 DX with thumb control water cooled and love it.
Hate to hear that.
The last SBC I built done that, when I re-done the build I put a roller cam and a set of roller lifters in it, problem solved. I'd have been money and labor ahead to of went that route to start with.