i did one of the first northstar conversions in a 55 nomad back in 94 , used a 700 r4 trans, designed a ring to mount to the flexplate and the converter to make up the space difference . it was no powerhouse , we had street and performance make a wiring harness .
the hardest part was hooking up the waterpump , because it was on the front side of the motor in the caddy ,then when you put it a rear wheel style the pump is on the back of the left head.
now there is a company in california selling performance products for them , so you can make some power .
good luck