Scat rods, no SIR's

No issue with a scat 9000 cast crank.

Look at Brians RHS X heads. They have made 500hp on 400+" engines. Once you buy valve train and have the speedmasters ported, you'll be in the same price range is my guess.

Simple cam, Voodoo hydraulic 404-405 comparable. Use a set of stock 273 style rockers.

The rest that you have will get you close to the 500hp range.