a converted hei ,was the timing curve played with??. maybe too much advance somewhere ?
i helped a buddie fix a 430 buick he had swapped into his 76 riv once. he had used the original 455s hei and was having ping trubile at cruising speeds. we went and got a adjustable vacume advance for the hei and fiddeled with it a bit and it solved the problem.
where did you get the hei conversion?? its a neat idea.