Unless the GMC bellhousing crossmember is different from the Chevy ones, you can look for a chevy cast iron bellhousing, and use your same trans. They also made that same style bellhousing bored out for the larger input bearing retainer of the sm465 (you should have a 420 right now unless someone has swapped it out, the 465 is what they started using after the 420 and probably what's mated up to that 454 with the aluminum bellhousing).
If it was me, I would:
find a cast iron chevy bellhousing that will mount to your crossmember
yank the v6
swap the bellhousing
drop in the 454
fabricate side mounts or a front mount for the 454
That way you don't have to move the transmission, cut out the bellhousing crossmember, shorten or lengthen your driveshaft, etc.
