Steve -
Metered head or plain-prism? Round or square eyepiece?
If it's the metered head (like the FTn) or the plain prism with round eyepiece, it will take the same screw-in diopter as the F2, FE/FM series, F3 (non-HP), Nikkormats and most other manual-focus bodies. The thread diameter on these is about 19mm.
If you have a plain prism with rectangular eyepiece, you'll need one of the old square diopters that slides over the eyepiece, or one of the eyecup adapters that is threaded to accept the round screw-in diopter.
Here's some good info and pictures here:
http://www.destoutz.ch/accessories_finders_acc.html
I've been looking for one of the rectangular diopters, or the eyecup adapter for months, and have only found one on eBay that they wanted $50 for. I'll keep looking...