When I got my car in 1982 the motor had almost all those same parts - Mullens ported alum heads, TRW pistons, 292 roller cam. When I had it rebuilt in 2000 the main changes were modern JE pistons, a 4.15 crank (477ci), and a Cam Motion 282 cam. It went from 650hp to 770hp, 550ft-lbs to 660ft-lbs, on the same dyno, with the crossram and carbs unchanged. I think those old pistons lose a lot compared to new, and those heads really want a stroker crank.

Edit: I agree the converter may be a problem. Just remember the best one for this combo will likely be too loose if you later build a stroker.