my bad, got you mixed up with another old man..lol
check out your HEI, the mallory that I had in the red truck had a problem that took me for ever to find..
it would do all sorts of things, stop running at top speed on the hi- way, miss after a few blocks, etc and then it would run fine for long periods of time.. it finally got really bad coming from a meet in Ft. Worth so I was determine to get it fixed...
turns out in was the electronic module in the dist.........replaced it, bought a spare, and never had anymore problem with it.. took about 5 min to fix..
a pix of what I bought I would check out the HEI.