ok, it dosent seem like money is an issue for you... buy a crate LS7 ($14,000) or KATECHs 427 shortblock (a bit cheaper). then buy a LG Motorsports cam, says that it will produce 600 REAR wheel horse power, mind you this is NA. no power adders what so ever. put a T56 behind it and you should have a car that makes at least 20 MPG.
if thats a bit much for you.. buy an LS2 ($6000), put heads ($1000) intake ($600-800)and a cam in it and you will have 500-600 hp, no joke.
you should very seriously look into the Gen IV GM V8s they are much better...