Swapping a 75 GMC 250 6 cyl. to a 75 Chevy 350 and want to know if the 6 cyl. bell housing and fly wheel will adapt to the 350.

All chevy bellhousings are the same bolt pattern(not sure about the diesels) but the flywheels are different.

yes. your bell will def bolt up and the flywheel should too, the only thing im not sure of if the size of the fly. with the 250 it might have the smaller flywheel that wont take the 11 or 12'' clutch, if this isnt the case, it should be a direct swap.
and this is yet another post that could have been answered with an ez search of the first page