Auto body repair & detailing: Frame Repair, rust problem, ace hardware

Question
I want to be sure I understand.  I am talking about the rear part of the frame.  The body mount bushing is attached through the hole in the frame to the underbody (actually bolts up through location underneath seat).  My rust problem is bigger than a 1 inch washer.  There is probably about a 3 inch diameter rust whole on the top of the frame.  Any change in plan or just a much larger washer.

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-----Question-----
I have a 1967 Firebird that I am beginning to restore.  The front frame on the passenger side (bolts to the support bracket under the passenger seat) is rusted through the top where the bolt goes through.  What is the best way to repair.  I figure cutting the end off and welding on a new piece is probably the best.  Any thoughts.
-----Answer-----
A shortcut that I found that works well for rusted out frame mounts is to go to ace hardware, and buy 3/4 or 1 inch flat washers. Weld them into place, and ream the holes the proper size for the bushing to fit into. It works really well, and the washers are about the same thickness as the frame. They are also the same strength. I did it on my 66 GTo, and it worked superbly. We are takling about the sub frame, and not the body itself, correct? If the body side is rotted out, then the mounting rails must be repaired or replaced. Bill

Answer
When I say 3/4 or 1 inch flat washers, I mean the hole inside of it. A 1 inch washer is huge, a 3/4 washer is pretty darn big. The frame bushing is around 3 inches across, and the sub frame has a hole nearly as large as the bushing, if not as large. In 23 years, believe me, I've been there, done that. The washer is quick and simple, and no one will see it. If you want to make it complicated, cut out a big square chunk of the frame, graft in a piece of steel, drill and ream a big hole into it. I don't think it will be as sound in the long run as my washer repair, and it will take twice as long. If the rest of the sub frame is solid, go with the washer. If it has excessive rot, replace the sub frame. I wouldn't trust repairing this to a novice, the sub frame suports the entire front portion fo the car. If it fails, the fenders can buckle, and possibly cause a catastrophic crash. Bill