[71GMC_3/4T]

Check the condenser inside the HEI unit. It should be down toward the bottom plate. I worked on a mid-70s pickup that would run great until you stomped it. When the vacuum dropped, the distributor rotated, and there was a broken wire on the condenser. It was about to drive me nuts until I finally pulled on the wire to the coil. The insulation stretched about 1/4", indicating the wire inside was no longer complete. It had flexed enough times to cause it to break, but would make contact until the vac advance moved it. This would be a cheap fix if that's all it is.