Second discovery is that an early Full Floater RM shock can be made to work. 2-way adjustable, 16mm shaft and 46mm body. Lifts the rear of the bike by about 20mm which suits me perfectly - it will match the 20mm rise from fitting the '84 KX disc brake front end.
Don't do this to yourself... For the love of god, don't do this to yourself.
I've just finished the conversion, and it was a total turd of a job... Ended up with KDX-C spring, modified RM spring seat, custom spacers and a lot of swearing.
I put it in on the expectation that it will need revalving, but now dearly hope that it won't, 'cause its a mongrel to get in and out of the bike.