to reply to all:
Nuke: Actually both are pretty close in weight... the 500 with an aluminum intake manifold will be really close to the lt1
aldo and shadow: remember this isn't supposed to be practical. it's going to be a weekend / rape 'ricers' and 'muscle' cars kind of car.
thing with the 500, is it will bolt right onto the engine mounts of the 6.0 i have at the moment. here's your trivia for the day: the four cadillac 'big blocks' of 1968+ were the 368, the 424, and 474 and the 500. all four engines had the exact same external dimentions... the block was identical, the only thing that was different was the bore size, etc... so the engine will drop right in the place of the 6.0, and bolt right up to the stock transmission (a turbo 400, one of the best transmissions in the world to date... still used on tons of hotrods). the biggest 'modification' i'd have to do would be replace the cooling assembly for the bigger engine. with the 350, there would be a lot more welding and modificating involved, and the cost would probably run me a couple thousand more.
either way i'd be suprised if either one ends up costing less than $8k