Stronglight is absolutely correct on this one. I bought a 1967 Paramount P-15 frame, fork and headset on eBay. Modern rear derailleur and BB (English thread, of course) screw right into place with zero hassles. 27.2 seatpost size, so finding a fit is easy. If you get one with rack mounts on the drop-outs, they are the same threading as the ones used today.

There is one possible surprise, though - if you get a touring Paramount frame, double check the brake reach. I found out the hard way that current-day "long reach" brakes - what used to be "standard reach," down to about 58mm or so - don't reach far enough. The lovely NOS set of Suntour Superbes I had reached to the middle of the tire sidewall on the back - not exactly what the doctor ordered. (The front is not a problem.) I solved the problem with some modern Dia-Compe center pulls from Rivendell. (If you do this, swap the pads for Kool Stops or Matthausers - the original pads are inadequate for the task.)

But wait, an even better solution is at hand: Riv is about to introduce honest-to-God long reach - truly, serious, 70mm or thereabouts long reach - double pivot sidepulls. I just saw (as in this past Saturday) a prototype mounted on a prototype frame, and it looks good. It'll be a house brand (I don't remember the name) and it will be made by Tektro just for Riv. The guys at Rivendell said they wil be available in a week or so. I think they said the price will be around $80 for a set of calipers, but don't hold me to that. Finally, serious stopping power and a good fit on my frame.