Can you clarify a little on your goal? First, do you have the time slip to confirm your past track results? How long ago was that and is that still the same short block you have now with the posted info?

If you went 112 mph in a typical 4000 lb race weight B Body with the old set-up, you were probably in the 450+ hp range, not less than 400 as you suggested. So are you looking for 500 hp (maybe 30 hp more that before) or are you looking for 100+ additional hp?

I agree with the others, 500 hp (lets call it 113-115 mph in decent air) with manifolds is not too hard - you just need a good plan.

Yes, CNC or port the heads. It makes achieving the goal easier.

I would get someone to recommend a custom cam for the application. Consider a solid flat tappet. Probably something a little larger than the Bob K cam. Bob's cam is good, but constrained by the Pure Stock rules.

I have no advice regarding the EFI.