Doug Fattic 

i recommend this patch should be brazed. I would use as sleeves 4130 tubing that can be bought at either Wicks Aircraft or Aircraft Spruce (Google them to find them). If your down tube is 1 1/8" in outside diameter, then the tubing you want to buy as sleeves will have an OD of 1 1/4" with a wall thickness of 0.058". This will fit perfectly around your down tube. This is much better and easier than trying to blacksmith something that doesn't quite fit. If you think you might like to try brazing it yourself (I don't recommend this on a frame you value), I would also get some feet of 1.125" OD with a wall thickness of 0.035". Before you even think of putting a torch to your real bike, you will practice a lot with the .058" as a half sleeve over a long piece of .035". Start with a small patch and if you are successful then go bigger. This kind of big sleeve takes some real skill to do and don't expect to be successful without practice. I've been teaching framebuilding classes for over 40 years and know many of my students would not be successful brazing a patch this large without instruction, a demonstration and then lots of practice.

Stainless Steel is much more difficult to braze and only experienced brazers should try. It requires a tighter temperature window (it has to be hotter than 4130 and can not tolerate getting overheated). If its temperature range is exceeded, then the surface gets corroded and the only solution is to take off the sleeve and start over. The bigger the area to be brazed, the more difficult it becomes to do it without overheating and scorching the surface so the silver won't stick.

What I would recommend is buying the 4130 0.058" tubing and make the sleeves yourself. This can save a framebuilder a lot of time. This takes some explanation and I'll wait to hear from you if you are really going to try doing it before writing a detailed explanation of what to do.