you can just turn the automatic transmission flag off of your existing tune, worked fine the last time i did it. i think one error code comes up, forget which one, and just turn it off. your idle tables and DFCO will need to be altered. make sure your reverse lockout thresholds are sane too i forget what GM put there. probably turn the skip shift junk off too unless you plan on doing bizarre shifting moves that might slam you into reverse.

i highly recommend the T56 swap. more fun to drive. more power to the ground. more better