That is a 1985, even the differential and brake panel are different, some 85 ATC250es differential will interchange with a 1985 only trx250a and some don't depending on the frame serial number which is FM063995 and up, so there were two different ones in 85 but that still don't help you, you need 1986 up and you have no axle tubes to work with, not saying it can't be modified to work, but I have never done that mod - I went thru 2 stock 300 rear ends already this year, didn't tear up any gears but had water in both and a bearing failure on the brake panel side of one, I am just about to finish a 1996 300 2wd with a 250 rear end and disc brakes on the front, this one I painted camo and got away from the red, my buddies liked it but I didn't and repainted it all flat dark green camo ,and it looks good, the other 300 stock rear end I have apart and doing seals and bearings one more time, collecting parts to do another conversion and all my 300 will be converted, seems I only get two years on the stock rear ends running them in water, my oldest bikes with the 250/350 rear end are over 5 years and still the oil looks great in them

Atc70 frame stretch