The parts inside most TH400s interchange, but aren't quite identical. A 425 engine likely has the weak 9 element sprag instead of the stronger 16 element with a 472, and fewer direct clutch plates. The plastic spring guide in the later may be curling up, but not the earlier aluminum guide. I have a description of some of the best (or NOT) internals on PHOTOBUCKET (search user bcroe), or email
bcroe@juno.com for a direct link. Bruce Roe