Engine misfire or loading up when opening the 4 barrel.

Maybe something in the new ignition parts? When under load, ignition parts, if not up to snuff, can show weaknesses. New or otherwise. I had a distributor cap that did that, years ago. In that case, it ran fine until I put the blower belt back on the Paxton. (87 5.0 Mustang) The extra cylinder pressure from the boost was trying to blow the spark out.
Tell us more about the combo, as TT5.9mag references.
Sounds like a nice car.