I don't know who told you 600 was the limit on the block, but I think they are wrong. I am in the 650 to 700 range with no problems in 3 years of running. I think I would put my money into a stock forged crank and a set of H-beam rods over the cast crank and rods. I wouldn't worry about billet caps as long as the rpms go to 1 are going to stay below 7000. Go with a set of forged pistons with a static ratio around 8 to 1 and you should be good to go with 7lbs of boost and the timing locked at 28 degrees on pump premium and if you want to get on it with 100 octane, go to 10 to 12 lbs of boost at 31 degrees and hang on.
Jack