I'm having a similar problem with a 390 that I recently dropped in a 76 pickup I have. I get it to shut off without dieseling but ping under power, or, after changing the tiiming have the higher speed power but the rumble afterwards. I'm beginning to suspect that I have the wrong distributer in the thing.
I'll be interested to hear the outcome on your dilemma.
I'll be interested to hear the outcome on your dilemma.