Agree with stlwagon.
If ya want 400 ponies just go SC.
The amount of $$$ to build up the 4.6L to that 400HP figure is pretty involved.
I prefer the alum head/ block because I try to keep cornering in mind as well. Going iron block and SC will put alot of understeer into the S197 without doing alot of suspension work to get some of it back.
Besides, the stock block is good for 700HP I have read, and some motor builders use it at that level.
My friends GT500 is pullied and CAI-tuned, dyno at 600HP.
If you keep yr foot out of it, it is very streetable, at WOT it is insane...but fun.
It is a pig in the corners though compared to the GT.
That said, even at 4000 pounds, the GT500 is surprisingly nimble in it's own right.
If you do a complete motor mod, good luck and keep us posted on results.