I converted my T-56 from an LT1 version to an LS version, There is a kit i bought from a company back east (Rockford i believe), bout 400$, includes a new input shaft, shims, and front "collar" which you'll need. I saw an article about this, either popular hot rodding or chevy hi-performance, which is why i did it...
http://www.gmhightechperformance.com...on_conversion/
I'm pretty sure if you have an t56 behind the bbc you'll need a different input shaft (as the final length is different) and collar, the lt1 tranny bolts up to a sbc, and the bolt pattern is the same for a bbc. You'll also need to confirm your bellhousing (you'll need an ls one obviously)