Bin the 4spd and drop an EA82 five speed in. Been done many times! Bin the EA82 engine if it comes with the gearbox… or use it as a boat anchor!
The EA81 4 spd is a pain in the butt to tear down and rebuilt. A specialist tool is required to remove the nut on the back of the pinion shaft that isn’t easily accessible. Fun fun. It can be done as others did it previously, just not easy. The 5spd conversion is easier in my book.
Cheers
Bennie