All the factory LA and Magnum heads like to crack...so investing in them is a waste.
So if you replace them with new cast iron heads from say EQ, then I would build a magnum engine, use a nice hyd roller comp grind, mildly port the heads and use 1.7 rockers (make sure the grind matches the rocker ratio).
Rpm intake, 950HP carb and 9.8:1 compression and it should make 400HP pretty easily.

A nice 3500stall converter and 4.10 gears should net you mid to low 12's with a good slick.