May want to ask Scott (blksmoklb7) I know he has put an lly turbo on an lmm. I believe the lly turbo is the biggest of the stock vgt"s. Lbz and lmm turbos are supposed to have better exhaust braking. Russell (yellowchevy) would be a good one to ask too. He could give you better answers than I could.
LLY is the biggest but they are inefficient to a point from what I've read, at least on the LLY so I'm not sure if its worth it. Anything bigger than that I'll have this thing blown up before 100k miles.
Inefficient? You have to mod the vane sensor connector if I rememeber right to make it work. Port the exhaust out to 3" throw a 10 blade turbine wheel in there with a billet or batmo out front.
Well I'll look for a used one that's not run to death and port it and slap it on. It'd be nice to have a dyno close to see how much difference there was.