Oh, yeah, I know how that goes. But I thought, a few posts back, you were considering 650b? Anyway, I'm afraid you have to put the costs in perspective. A pair of rims is going to be easier, faster, and much cheaper than a custom frame, or even a customized frame. You can doubtless sell your 700c delgado's on CL or ebay, and build smaller wheels, faster and cheaper than getting cantilever studs brazed onto an existing frame.

As for customizing a frame, don't do it unless you've already tried the exact geometry &c you're going to end up with, lest you find yourself heavily invested in a unique frame that doesn't suit your needs as well as you'd hoped, and that no one else will want either. And since you seem to be one of these guys who likes to experiment with changing this and adjusting that almost ad infinitum --in short, someone like me-- I don't think any kind of custom or customized frame is a good idea.

Have you already built the rear wheel? What's the hub?

As for the Trek 720, be sure you're correctly distinguishing between the early 80's 531 tourer and the early 90's hybrid. They were totally different bikes. The former had horizontal drops; I know nothing about the latter. The former is considered by some to be a top-notch tourer along with the Miyata 1000; others find it inferior to the Miyata. Now I don't know anything about the Miyata, but chances are good that if you didn't like the Miyata 1000 you will be equally unimpressed by the Trek 720 tourer. And anyway, the 720 tourer does not have such generous tire clearance as you'd think. I have 37's on mine, and it's a very tight fit.