Now you can't be expecting to have a 40-45mph mud machine as if you want it to go fast alot you will need to remove the reverse chain as it's the weak link and will break before anything else and if that happens you'll be looking for a new transaxle. So if you want to mud/race id suggest having two different machines one for each, as mudding you don't need 40-45mph speeds lol. Same with the brakes, if you want to go them speeds you'll need brakes, meaning you shouldn't use the stock brakes on the tranny because in the case it ever explodes you won't have brakes at all; so shaft brakes are better.
Remove the old grease, clean everything with a good solvent, EXCEPT the brass bushings if present, wipe them out with a clean dry rag so as not to remove the impregnated oil finish and remove the reverse chain. Not sure if the 600s have them but I know the 700s do, but theres a little screw in piece on top of the tranny there it looks kinda like a sensor (neutral switch), remove that and put in a 1/4" brass barbed nipple in it and run a vent line. You'll need to cut about 3 threads off it for it to fit down all the way, and put a oring on it to seal it up good. Then you can weld the spider gears to make it posi-track.
From there use combination of gear lube and 00 grease as lubricant, be sure to pack a little grease in the tubes around the axle, it'll help seal the axle and prevent the lube from leaking out the axle tubes.
Now you can do the same for a mudder just don't have to go so into it. If your just going to mud, going a mudding speeds (wouldn't imagine that'd be 40-45mph lol) then you can leave the reverse chain in there and use the transaxle stock brakes as I wouldn't see yea breaking it. If you did break it from mud riding you wouldn't really have to worry about flying off it when the metal gets in the gears and locks up the tranny going muddin speeds.
Anyways best of luck to whatever you deside to do