I put a couple of T5s in my 40 chevy coupe years ago. It is a major undertaking. First attempt was with an s-10 but ended up with a non-world class out of v8. I had the body off so it was easier work on the changes.
One thing that seems to be missed is lack of room between the floor and the cross member. The T5s are much deeper and longer than your current trans.
I ended up cutting out the mid section of the cross member but saving the triangular ends that hold the bake and clutch and add strength to both frame rails. When the piece was removed the two triangular structures rotated or sagged down. A new cross member section was made to bring triangular ends and frame rails back to their original position and to go behind the longer T-5. It is made from 1 1/2' angle iron shaped like V with two straight ends extending out from each end of the V - kind of a flying V shape. I bolt it to one side and jack it up into position and and bolt up the other side so the rails and triangular ends are back at their original position.
