[jocko]

a TH350 slip yoke is a TH350 slip yoke, so if it's the proper slip yoke, you should be fine as far as outer diameter of the yoke vs seal lip dia. Did you look before installing to see if it was a slight interference fit on the yoke? or could you look now by inserting the yoke a bit? Should be fine. Speedo leaks are maddening, but, if you clean and wait to see where it leaks as mentioned above (as in literally stare at it and don't drive it - or at least don't drive it before you check it again, because the wind from driving will trickle the leak back toward the tip of the tailshaft and it will be hard to tell again) you should be able to decide which it is. If it's definitely the rear seal, the only advice would be as already mentioned - thoroughly clean inside tailshaft lip, tap it in, then rtv it a bit around the outer dia of the seal/tailshaft mating circumference. Don't put a big glob of rtv inside the tailshaft then whack the seal in - it will dry, detach, and end up back in your trans somewhere you don't want it to be.