Dont think so....
I'm not sure,but I think the TH400 isnt the same as ANY TH350,no matter what length "tail",but they might be close..(*most*not all, Gm trucks use the 9" tail TH350,the "medium" length tail")--.you'll have to move the crossmember to line up with the TH400's tail mount no matter what,(they are in a different spot--and you may have to drill the holes for the crossmember in the frame)..- you could use "conversion" U-joints IF the old driveshaft will be the right length..TH400's use bigger U-joints,so the driveshaft itself is bigger to accomadate the bigger u-joint caps..some trucks used two peice driveshafts and a center support bearing with TH400's..
I usually just get all the related parts at the junkyard off another truck that had a factory TH400 in it...its a hell of a lot easier..I realize you probably want to swap in what you have lying around (and who doesn't!

)--but its not always that easy...its usually bad enough just putting back what was there already!..
A TH400 is a beefier tranny and more desireable,but its a lot of work to swap one in unless you have a "donor" truck with everything you need,like the kickdown switch,wiring harness,driveshaft,etc..A TH350 isn't junk,they hold up pretty good if you run a good size tranny cooler..
