Is a 12 bolt rear out of a 66 buick rivera the same as a 66 chevelle?

I believe that would be a BOP rear and is not the same.

Riviera is also a big car, like the impalas.

Actually, the Riv isn't much at all like an Impala. Totally different body design.

I'm thinking that in '66 it was a RWD version of the Toronado "E" body.

I think the axle is not at all related to the Chevy 12-bolt. The Riv guys I know are always looking for gears and posi units to fit that case--and they don't often find 'em.

You're right, it is similar to the Olds Toro, Chevrolet doesn't have a similar model. It is still a Big car, so the spring and control arm mounts are completely different, and won't cross over to the A-body cars.

Yeppers, I picked up a '64 Riv at auction sometime back. Everything about them is screwy and relatively hard to acquire both parts and info. Even if it was dimensionally compatible, the Riv has an "X" frame, I'd imagine that the rear control arm brackets on the rear end are configured differently.