[mylittletony]

I have had this done, but using a donor car (as you know!)

The safest way is to get the box, cross-member and complete prop as a package, then there should be no compatibility issues. I can't comment on joint types, but the front section at least is different due to the boxes being different lengths. Hence easiest to get the whole caboodle.

Mine needed a new slave cylinder in the end, so may be worth factoring this in at the start. Also a new clutch while it's apart is sensible!

The ratios should be ok, the 5th (or OD on M46) is in addition to the original 4 gears. i.e. 1-4 have the same ratio whichever box you get (there are some short 1st gears as the exception to the rule)

As Clifford also says, the speedo pickup is important:- rear axle sensor was introduced in '85. There are ways around this (obviously including an axle with a sensor), 81-85 you can swap instrument clusters and pre 81 you can build a hybrid cluster but it's quite involved, I'll create a thread once I've finished mine.