I think I may have an idea.

If I take a smaller frame, and remove the bottom tube, and cut the top tube in half,
And then add 3 or 4 inches to the top tube with a slip-fit tube that fits tightly around the original top tube ends,
And then put in a new bottom tube from crank to headtube,

It would only be 4 welds. The top 2 welds would be simple to do well; If I use an elliptical tube for the bottom, the whole mess would be even stronger than the original because the welds from bottom tube to bottom bracket, and bottom tube to headpiece, would be longer. I would MIG weld it.

I wouldn't have to fabricate chainstays or mainstays or mess with geometry or fork angle or any of that hard stuff. I'd just be lengthening the top tube and replacing the bottom tube which would also be longer and have a different angle, defined by wherever I decided to put the head tube. No fancy jigs, either; get the top tube lengthened/welded so the seat tube and head tube are in the same plane, and I've got it.

Voila! Custom frame. Just need a lightweight, well made cromoly donor.

The search is on. Let's see how cheap I can do this. If I find the right donor, I won't even need to buy components like wheels and such.