Scott, if it helps, I have an early Frey, from his days in Tim's shop, no joint collars. I have an original 314 with a 3/8x10 with a black collar that I have tried on it, that I bought for another cue. It matches up very well, with a very minimum of diameter difference...less than 1mm overall.
The Frey/Pred combo is OK, but you will lose the Frey hit, which is very firm from my experience. My Pred. has a Moori med. on it. I'm still looking for the perfect tip for the Frey, had a LePro on it when I bought it second-hand, tried a Sniper, but it felt like a dead hit, but did spin the ball better...now I have a Super Molavia med...kinda feels too firm. Scott, you should also weigh your Frey shaft, because, in my experience, the Pred. shafts tend to be consistently lighter than the originals, which will affect the balance of the cue...it will move the balance point closer to the butt. As far as diameter goes, although I don't have calipers, I just tried the Pred. on...the Pred's joint diameter is slightly bigger, but barely noticable on mine, and I don't cue there anyway, so no matter. Anyhow, doesn't hurt to have an extra shaft, and there's nothing really wrong with that combination...but you will probably find yourself going back to the Frey eventually....Good Luck, I'll let you know if I find the perfect tip combo for the Frey.