74 Swinger 5.9 Magnum with FAST 2.0 EFI

Wait, so you swapped from a factory roller cam setup back to an old MP flat tappet grind? Why not just throw a Magnum roller Hughes Whilpash cam in it if you wanted the rumpity rump? I'm confused I guess. Never heard of anyone back fitting a flat tappet into a Magnum?