Sounds like you have a bad ground. Are all your frame to engine grounds good? You replaced any/all grounding straps when you swapped the tranny?

Use some jumper cables but use one side, connect one end to a good bare metal part of the engine then the other side to a bare spot on the frame and see if that gives your starter more power.

I've recently discovered the value of doing voltage drop tests to diagnose starter problems, but its difficult to explain and you need a good multi-meter.