can the rear axle mount be repaired?

If it is toast, I would think a replacement mount is in order. Is the mount part of the unibody? Are other parts of the car rusted as bad. Repairing a 31 year old rusty Cutlass is not a good idea. Personally I'd be looking to replace the car with something newer.

I agree, the structural integrity of the vehicle may be compromised making the vehicle very unsafe. In my opinion it's not worth fixing. Move on to something newer. Hope that helps! Jim

If the mount is toast then so is the vehicle. You might part it out then tow the rest to the scrap yard.

Without a photo or two it's difficult to guess. One persons idea of bad corrosion is surface rust with bubbling paint to another. If this car really means something to you and is not a complete rust bucket take it to a body shop or a restoration shop and let them have a look. Beware of having them talking you into a car repair that may not be worth fixing.