Replacing automatic transmission with manual transmission

[Vic Norman]

Hi - I'm looking for some words of wisdom from the list...

I have a 1991 Audi 100 (non-quattro) with a 4-speed automatic transmission.
The car has nearly 200,000 miles on it and the transmission is pretty much
on its last legs. Since I dislike automatics, this seems like a good excuse
to replace it with a manual box.
My question is - can I use any of the 5-speed boxes that were available for
the 5-cylinder Audi 5000/4000/GT Coupe over the last 20 years? Are some
better than others? Are there major changes in gear ratios and final drive
gearing that I should worry about? Can I expect problems with driveshaft
interchangability?  Is it likely to be a major problem to replace the
2-pedal auto setup with the 3-pedal manual?
Help with any of these questions would be greatly appreciated.

Thanks in advance
Vic Norman   1991 Audi 100