For what it's worth, there have been a few LS1 bottom ends for sale on e-bay....one guy in Michigan (going by the tag 'Michiganmotors') and another on MO (who's name I don't remember.......)so if you are willing to build up your own the blocks are available. Heads are there too..T56 tranny's, on the otherhand, seem to be scarce on e-bay, and I'd hesitate to buy one sight unseen for exactly the reason everyone is posting here..there are bolt pattern differences depending on the source.
The best solution is to find a mated pair from a local yard.
In my conversations with HP Salvage, they seem to be able to get ahold of whatever you need - for a steep price - but you get the ENTIRE package: engine, computer, accessories, wiring harness, etc - all which make a bolt in possible. In my salvage yard searches in Chicago, I can find a block...or maybe a tranny, but nothing as complete as what HP offers....and I think that is where he may have a lock in a niche market. $5800 bucks is alot for an engine/tranny combo - but in the end it may be worth the price as you don't have to piecemeal a drivetrain and engine management system for your project....
...And good luck getting a yard to guarantee your motor past 30 days! .....In my example, these are going to be the last parts I purchase/install due to any warranty clicking away at the time of purchase ...