if you want my .03 you should sell the LS1 you have now for cash, and use a bare block or a motor that is blown for your mocking up. with that much hp/NO2/boost you really need to step up to a cast block. its not that much heavier and you would probably be happier. a C5R block or LS7 block would be something you should look at also.
personaly i also think that you shouldnt do a really radical custom, yeah its cool, but if you are going to go and replace just about every single part on the car and make it custom, why not make a completely custom car that dosent look like anything else... just drop the LS1 you have in there as the car sits now, just making it nice and clean. depending on what cam you get you can get 400 on up rwhp out of a stock head/bottom end LS1. when you want something a little more... swap heads and go to a FI cam and run low boost on either a procharger or a turbo set up..
there was one guy on LS1 tech, had a A4 3600 lb Firebird with a forged stroker motor, big cam and aftermarket heads and he would do LOW 9 seconds on the bottle... LS1s have pretty much unlimited potential, only limited by the pocket book