I agree, the damage you have is not uncommon, I bought a junk yard rear just to get the gears and had a similar problem.
Yes, you can get about any rear end you want in there, but do you need to? The 7 5/8" rears will handle close to 300hp in your car which if you are doing a pure restoration should be sufficient. With a spool or limited slip and a set of good slicks, you can break them too, but that combination was not availble from the factory. I do have a factory posi for that rear end sitting in my garage which I would sell to you for not much if your spline count is right. I was going to use it until I looked into what they will handle and found that my 450hp is a little out of its range.
If you need to have the bigger ring gear, then I would get hold of a 12 bolt and narrow it to what you need. You can do the narrowing at home with a welder, I can show you how. You buy the axles for $225 and you will have enough strength to handle a big block.