flywheel question- one piece seal vs. two piece seal.

[crossy]

Hey guys I have a bit of a problem solver question here. I'm trying to install a 1979 350 in place of 1986 engine. the 86 engine had the one piece rear main seal. the truck is a stick shift truck and has the 13.5" dia. flywheel in it. the 86 flywheel will not fit the back of the 79 engine. the bolt pattern is totally different and and the center hole on the 86 flywheeel is smaller in diameter than the back of the 79 crank. I thought i could just get the crank hole bored out on the 86 flywheel, but the bolt pattern is different also.
QUESTION is simply this. can i just install the 85 and earlier style flywheel ?? the truck has the hydraulic clutch. I was pretty sure the trucks started using the hydraulic clutch BEFORE they went to one piece main seals so this should just be a simple swap right???
At the present time i dont have an early flywheel here(usually do), but i want to be sure before i purchase one somwhere.

thanx , crossy.