I try not to post in threads that might just revolve around what I do for a living. However IMHO, I would stray away from an alum block, although they are great on weight savings aluminum moves a lot more than iron does and the more movement in the block the less it will seal which in turn means more power loss. The alum vortec blocks cant take much of a bore unlike the iron 5.3 and 4.8 can be bored to ls1 specs. Ls7 i would stay away from due to the Siamese cylinders and thin walls (well for boost they are too thin)
Ls3 blocks are nice for boost along with most any other ls block. just be cautious of vortec alum blocks.