About the brakes, and wheels too, Trek sized many of their early frames for 27" wheels instead of 700c. It will make a difference when you select a brake reach when replacing the calipers. I personally like the Suntour Superbe. They are very positive when used with Kool Stop salmon pads. The Superbes come in short and slightly longer reach so be aware when selecting. Since your hubs are probably not original (assuming the 130 spacing) then you probably do not have the original rims either, so probably 700c.

Recently I met a fellow with a showroom stock immaculate 714 model from about 1981-1982. His bike was absolutely original and looked like it had just been set-up out of the box. I noticed that there was a large clearance between the front tire and the fork crown which made me realize that it had 700c wheels. There were drop bolts on both the front and back brake. My conclusion was that Trek had done that originally, perhaps as a 700c option. Your 720, being set-up for calipers, could be similar. Just saying, it's worth checking.

Jim