Learnin’ all the time with this project. So I tried karthiefs suggestion.
Out of all the chain parts stores (except NAPA, I didn't try them) , only Autozone will sell you one plug wire. Kind of a tricky thing to navigate on their site, you need to know the length (28DQ for the number 8 cylinder on a stock car!) but can be done. Even the clerk there was like “well, we don’t sell many of these single wires.”. But for chasing down a problem for cheap ($2) I was glad for the option. The guy was like " This was a headscratcher as I tried to fill your order ( I did "Pick up in store) because we don't sell many of these single wires."
The issue is not resolved though. I just ran it about 70 miles, mostly highway and it's still not 100%. But the misfire happens much less often now and the idle is much smoother. Last cylinder balance test was all 90's, but I think the issue is more intermittent than it was previous. I'm considering replacing all the plug wires with a new set, even though the set I have is a year old (Motorcraft).
I might just end up re-doing the fuel system anyway. The sending unit appears to have stopped working because the gauge never leaves full now. It wasn't accurate before, it certainly isn't now. If I have to drop the tank for that, might as well do other stuff.
91GT, no smoke on start, but the start can be a bit rough. It starts, but not always smoothly when cold. Hot starts, no problem.
Fuel pressure is around 35 psi at the rail, I'm not sure the accuracy of the Harbor Freight fuel pressure tester. Max is 39 I believe I read in the service manual.
Thanks for all the help so far.