Welcome to the Jungle, DG. Whereabouts in western MI are you located?
Used to spend a fair bit of time in the Grand Rapids/Holland/Wyoming area myself about 10-12 years ago.
I don't know that there's any real comfort difference between the earlier and later model seats, as they're all universally recognized as being pretty sucky, although livable for short-trip duty.
And, no, the '79-80 seats are not interchangeable with the '81-84s: that would be too easy!
If you're really looking for comfort, you'll have to either go aftermarket (hard to find), or get a rebuild done on your stocker.
Welcome again, and come back often

Steve