Yeah, 79 isn't the best of years to monkey with.
It's probably worth more as a "original survivor" now - to mess with it might be counterproductive as you'll spend a bunch of cash to never re-cover it. It is only original once as the saying goes.

But, it's your car.. your wallet. If you do swap trannies, keep the old stuff of course, like you said maybe the next owner will want'em ? Probably! LOL..