fede

Unfortunately I won't be able to get any more pictures till February at least but I'm reasonably clear now about the issues with having that welded.
As to why it might have been done I'm not too sure, the 2 explanations I came up with are either a very crappy repair job or as a way to stiffen the rear suspension for performance reasons.

MelWff

the normal way to stiffen that crossmember is to remove the bolts and use round aluminum blocks under the bolt heads, not welding. See link below.

https://willcoxcorvette.com/corvette...cing-kit-63-75

TCracingCA

The aluminum discs are more safety, but also give a bigger footprint to prevent that crossmember from rocking on the rubber donuts! For street cars, you want some isolation by use of rubber or a softer material for street! Usually welding that crossmember is to remove the rubber and bushings to move it upwards, essentially lowering the car! The rear crossmember welded substantially reinforces and strengthens that rear frame area. It also eliminates any movement of the diff that could translate out to suspension and handling and gives stability to the drivetrain component relations under hard usage! I hate to give secrets away, but on my racecar to shed weight, that heavy piece of metal is gone except for just the rear diff mount area welded up!

TCracingCA

I do that with chromoly to chromoly or chromoly welded to steel, but never needed to anneal or normalize steel welding on steel! If you tig, then you never need to do any of that!

ignatz

That suggests there wouldn't be room in a welded crossmember for the tools I've used and showed here.

TCracingCA

Generally not! A lot of racecars just have removable panels or nothing back there to get all into the rear end area from above! That crossmember doesn't look like it was moved upward! My question on old race or old autocross cars when I see something like this, there are more mods that give a clue to maybe some history!