none of those motors will be a straight bolt in. just changing the motor alone, won't solve your problem, you would have to change out everything for anything other than a 350 motor ?, as every wire harness, ecu,c.d.i. meter display, all of it..wont just all go together like you want ?. your best bet ?, just find a trx400,trx450, or trx420..and go from there.
Going to be a biggggg job. But there has been quite a few 400/450 motors swapped over the years. Don't know what's all involved, but would be a chore for sure.
There is a few guys on FB that do GR's. Can't remember all the names, hate to leave someone out. I would suggest just going on the Rancher Club, or Foreman Club etc. and asking there or doing a search on the group page.
Don Oglesby does gear reductions. He's on bookface.
I agree, if you're trying to swap motors, better off just selling your 350 and buying a 400/450/420. Getting the motor in place is the easy part. Getting everything else (wiring, driveshafts) to work is gonna be a LOT of work.