[mr48chev]

A few thoughts.

Distributor timed right?

If you have the cap off the distributor and can turn the crank back and forth by hand how much does the crank turn backwards before the rotor moves? If you pull number one plug and bring it up on compression and line up the marks, is it in time?

One thing most guys do when they swap a V8 into an older truck is put a new timing chain and gears on it when they are checking it out but if it was pulled directly from a donor and especially a low mileage donor it may still have a plastic tooth timing gear in it. Even double roller chains wear out after a while.

One thing to think about is always review any and all changes you made recently before it started acting up. Quit often the littles change will end up messing with you. Also don't start swapping parts before you actually test the parts you think are suspect. A lot of times we change stuff because that was wrong with Uncle Bill's car when it acted up so it must be what is wrong with mine.

The pickup coil wires are famous for breaking or partially breaking and letting the rig run but loosing contact with the broken wire ends when the advance plate moves. I got burned and a lesson on that one a number of years back. That was before I was teaching and had the pamphlet that I posted the link to. The good was that a guy in the shop had been around that exact issue before and said that was most likely the problem and then showed me exactly where the problem was.