the block you want is any bigblock with a casting number thats 512, if its a four bolt then congrats, if its not then upgrade to splayed mains. the crank.. well i would pass up any ls-7 crank for a new SCAT or eagle crank. you can even get the eagle internally ballanced. with regards to the rods then go eagle or simmilar. take a look at this link:
http://www.greenwoodchevy.com/after.htm
they sell dirt cheap high quallity stuff. and lastly if i were you i would buy a merlin or bowtie block and get the bigger bore... its free horsepower!! i mean it will cost you the same to build a 540 merlin as it will to build a 454 LS-7 (and the stuff carries a warranty, unlike 35 year abused chevy junk)
and if you want to go overboard, then buy a merlin lite alyminum block and slap on a pair of AFR 335 CNC heads and a roller cam nad you will have a motor that makes 750 HP on pump gas, now try that with a LS-7