The torque your making now with the big block will make that converter stall much higher and seem alot loosier than behind your smallblock, or you might have just broke something in it.
I'd pull the trans pan and look for metal which would be a good indication that something let go in the converter.
I would have a new one built especially for that motor rather then trying to tweak your old one.
Also dont forget to figure in the weight difference between your sbc and the bbc which effects things like weight transfer, center of gravity etc. Maybe a chassis adjustment is all that's needed?
JMO
Goodluck, Charles