I fitted an R80/100R shock to Jills 100GS. This was to lower the bike so she could reach the floor. We considered fitting a main stand from an R80/100R but in nearly 4 years haven't done this yet. It is a little more difficult getting the bike on it's main stand, but not that much. I can do it on my own quite easily.
Moto Bins offten have these in their bargain page at around £20.
Regardless of the length of the shock. When you are sat on the bike it will be shorter. Therefore you won't have any UJ issues.
However in answer to your question the Ohlins is longer than the standard shock, which I believe is 485mm. Jills 100GS came with an ohlins which had been on the bike for several thousand miles, with no damage to the UJ.
A slight hijack of thread here, I have a 1990 R80GS with the original white power shock, the bottom bearing has failed, and I am at my wits end trying to source a replacement. It is a spherical bearing that is 15mm id x 25mm od x 10mm wide. any help appreciated.
Steve.
ps. apparently whitepower are no longer in existence.