Frame connectors are easy to make and a LOT cheaper than what USCT sells. Not to take anything away from the stuff, but nothing they sell is rocket science. Full disclosure, I have some of there stuff for my 70 cuda. Our 65 valiant has home made connectors and they are perfectly fine.
As far as they rot is concerned , you're not gonna know until you cut it out. I can tell you, it's going to be farther from the opening that it looks though. The thicker metal, the more damage had o be done to make a hole like that (from the inside). I'd make a layered tape template to capture the shape of that whole area, then cut a nice rectangular peice out. Keep cutitng until you can't feel the rust on the inside. Then you can measure the thickness, and make a patch using you're template. The rest is treating what's left and a little welding. Done. Metal is forgiving. You can make mistakes and fix them easy enough.
Good Luck!