Having built a couple of BT strokers, I know I stopped at 4.5" stroke because of the difficulty of coming up with a stroker piston for longer strokes. I Might have gone further if I'd known the Briggs would work. Ah, the road not taken.(The faster road not taken)
Evo (3.5 060 over ULH) Corvair, etc is all I have dealt with, as far as pistons for the big 80 go. What have you tried?