If your planning on using the post 89 G50 you will also need to modify the end caseing as the later one have a huge mount on the front of the end case.
To do this you have two options. Buy an endcase from an 87 - 89 G50 but they are $1000 new and almost impossibe to find used I have just gone through the same thing trying to fit a G5003 into my 1972 car. The other option which I chose was to cut down the end case and make a mounting bracket to take the tranny cradle.
See my G50 conversion thread -
G50 Conversion Project Begins
Good luck Steve