if it's that old,
If the tape's THAT old, it may very well be acetate-based tape. Hold it up to a light, and if the glare off of the tape packing is lustorous and "bronzy", and shiny, it's acetate. if not, it's standard Mylar/polyester.
Acetate tapes are very resistant to stretching, but are more brittle than mylar/polyester tapes. They don't suffer from sticky shed, so don't bake them, there's no need to.
If the tape is fringed or warped along the edges, you may have to increase tape tension just a hair to get good head contact. but don't over do it, or the tape will break right in half. (Nice part about that is that if it does break, a splice will repair it pretty accurateley as it stretches very little. It shears right off.)
At least that's what I hear about acetate. Good luck w/ that.
Might also want to make a backup copy, just incase you can never play the tapes back again.
-callie-