David,
Didn't mean to pirate your thread here. Welcome to the LS1 ranks! Hope you are very talented and have lots of $$$. It is going to be a great ride once you're done!
Rufus,
I saw a T56 with 40,000 miles on it yesterday on LS1tech.com for $1,000.
As to your question about the Tremac - I assume a T-5? I don't know for sure - which manual tranny does GM normally provide with that engine?
Changing transmissions will require reprogramming of the ECM but that is all. besides the normal stuff with the torque plate, bell housing, clutch, etc. My engine originally came with the 4L60E. It now has a T56 behind it and it works fine.