T-56 Magnum to LS3 - new input shaft?
 

So I was told by some buddies of mine that I can't just get a bell housing to mount my T-56 to my LS3, and that I will have to change out the trans input shaft because the one that is in it will either be too long or too short.....cant remember which. I haven't heard of anyone doing this. I thought I could just get a quicktime bell housing designed to mate the two up and be good. Can someone tell me if I will need to change the inout shaft?

Not only does a t56 magnum and ls3 go right together if you use an Fbody bellhousing and throughout bearing you won’t have to worry about shimming it. Exact setup that I have

A t56 viper box would need a new input shaft .. the t56 magnum was/is designed literally for these kind of swaps

Yeah, depends on the T56. Early models not originally installed behind an LS engine require a new input shaft, front cover, and bell, which requires some setup work. Other T56 boxes, as Ryan said, require a different input shaft as well.
What T56 do you have?

I have a Tremec T-56 Magnum from American Powertrain that I got 4 years ago when I installed it behind my current 350 small block chevy.

Same here.