Will a 85 gmc truck transmission fit in a 89 gmc truck and what do you have to do to make it fit?

If it's a manual transmission, you'll be talking about putting an SM465 to replace an NV3500 or NV4500... I'd be doing the opposite swap, personally, as the SM465 is a direct drive transmission with no overdrive gear. It can be done.. you'd just have to use the NV bell housing, and you might have to replace driveshafts with ones fitted for the new setup.
More the same with the automatic transmission... there's no reason to replace an overdrive 700R4 with a direct drive TH350.