Someone else who knows more about this will have to confirm it, but a clutch-to-flywheel problem comes to mind. I'm thinking a different clutch is needed. I read this somewhere, but I don't have a manual, so I didn't pay much attention. Hopefully someone wif the info will be along soon...
At the very least, you'll need new motor mounts...and I'd suggest the ones from Wild Horses. As for the tranny bolt up...dunno about the manuals, but to autos it's a direct bolt up.
Bellhousing will fit either 351M, 400, or 460, clutch will probably interchange too, depending on what year of truck you are working on. Is it an 11" clutch, and is it mechanical or hydraulic?
Be sure and take a look at L & L Products as well. Their brackets will make the swap a thousand times easier. My 460 swap was simple using their brackets and motor mounts.