I dealt with a close version of this quandary doing restoration work on my 41. I solved it by picking up a better frame. My logic was why would I put thousands of dollars, and a couple thousand hours in restoring the rest of the car to drive it around on a questionable frame?
Don't be too surprised if when you start probing the rusted areas they show hidden rust through. How much time, effort, and money have you budgeted to repair your frame?
In my case the front cross member bolt holes holding the A-arms were rusted out and elongated to the point I needed to use oversized washers to hold the bolts in place. The car had been in a front end accident where the frame had been cracked open and rewelded shut. There was not a lot of rust damage but the rest of the frame looked like it was bottomed out in a field of boulders. For $100 I won a bid on ebay for a complete frame with all its suspension. The frame was from a guy Hot Rodding the car and was from Arizona. It cost me $400 to go pick it up.
The frame is like new. I had it sandblasted and have it all painted and cleaned up ready to put my body on when I finish restoring it.
Good luck, Mike
P.S. I have spent a lot of time cleaning out 2 frames. You should not introduce water into the equation. It is easiest done by using a good air compressor and a lot of probes and lengthy bent brake line tubing. Make sure it is a windy day and you have a very good respirator on. The things that have lived in frames, and died there, and chemicals picked up from dusty roads will give you a good respiratory infection. I have parted out several cars so I have "been there done that." A "barn find," can be especially toxic.
Here is a series of past posts on my frame experiences. Have you learned to use our index of past posts? If not let us know and we can explain it.
Painting frames and suspension parts Part 2 Painting frames... Part 3 Painting frames... Part 4 Painting frames...
