The 4sp is a pretty good tranny. Never had one myself, but we beat the snot out of a couple of my friends trucks that had 'em in college.
The 3sp (on the tree) is a great tranny, a little cluncky to shift, but I've never heard of one breaking, I've had 2 of them, my current c10 has one, 168,000mi and not a lick of trouble with the tranny.
I believe the bellhousing in all cases is the same. I know my '86 C10's 3sp bellhousing fit's up to a SM465, I believe the 4sp bell does as well.