I've been sitting on a 71 Riviera for a few years now. Same old song and dance. If I have time, I don't have the money and when I have money, I have no time. Now we sold our house and I just moved the car to the new property. The issue is that it was sitting on the left rear frame due to a tire that went flat and once I got it on the trailer, I realized the inside of the frame rail is just gone. I'm sure I CAN fix it but the question is, do I want to? I did some searching around here and didn't find what I was looking for but I'm under the impression that no other frame is going to fit under this car correct? I'm not having good luck with these cars. I keep buying them with my heart rather than my head.....
Go online and try to search for the 1971 Buick Chassis Manual (in google add filetypedf to your search to get the content). (you can buy a CD copy as well). There will be a section on frames -- you can see exactly what the Rivera's frame looks like (exact dimensions) and how it compares to other Buick frames. The issue is really what the rest of the frame looks like -- any better??? It seems odd that it would be just one spot -- get a chunk of pipe and start tapping -- you'll see what is what... If it's just one area I'm sure you can find someone to repair. Based on my own painful experience, if it is extensive it might be cheaper to find an existing frame and try to swap out a section VS try to piece in repairs. Good luck...
If there's no other source for a frame part, you could go to a local machine or metal fab shop with the dimensions from the manual and have a splice-in piece made (I did it for the rear frame crossmember on my Wildcat). This is dependent on the condition of the rest of the frame, however.
