Hi Dave, I installed an 850 transaxle in a 600 once. It was a LOT of work for little gain. Trevor has provided some good info. on the conversion. You end up with synchro in 1st and you can use a stock 850 motor without having to reverse the rotation, but... You have to butcher the chassis and do a lot of cutting and welding to get it to work. It is quite difficult to get the axles aligned, and if you don't, you'll be eating up the rubber-filled couplings every few days. You MUST relocate the rear pickup points for the trailing arms (due to 850 case being wider), and you have to consider what this does to the suspension geometry and handling.
Personally, I'd rather invest my time and money in reversing the rotation of an 850 motor any day over putting in an 850 transaxle.
Gil