You will probably find that your bits will fit any OIF frame (650 or 750), more or less. That's the nice thing about Triumphs, most bits fit most others. It's also the bad thing about Triumphs because there are subtle differences that come back to bite you later.
Bear in mind that all the pre Cooperative bikes (all 650's and the very early 750's) were right foot gear change, left foot, drum (conical hub) rear brake, so you need somewhere to hang the rear brake pedal and a suitable swinging arm for the drum brake. I think that the brake pedal went on the drive side rear engine plate but I'm not dead sure. Check the parts books...
The 650 frames had a habit of cracking around the swinging arm pivot, which led to a really anoying oil leak; you had to dismantle the whole damn bike to fix it!!!! The 750 frames are much better in this respect.
Another thing to watch is the fact that the early type seat hinges from the other side. Also remember that the early frames had a very high seat height. Not the end of the world but a bit of a pain in the butt, literally.
Post 75/76 750 frames are much more common anyway, so that's probably what you'll end up with unless you are a slave to originallity.
Search Google images for Triumph T120v for pictures. If this all sounds a bit daunting, don't despair. There's loads of information and spares available for the late Bonnies. A decent 650 Triumph should be good for a genuine 100 MPH (although not for too long or too often) and the OIF frame is a really great handler; best of the lot IMHO. You can build yourself a great bike from this lot, anything from a chopper to a cafe racer and you won't get too much bother from the "spoke counters" with a late model.
If you want to see where the brake pedal fits, see
http://motorbike-search-engine.co.uk/classic_bikes/triumph_t120v.jpgGood luck with your bike. The Triumph "project bike" is part of a noble tradition...
