If I'm not mistaken, the 78 Riv or last RWD Riv is on a B-body Chassis.
The Caprice/Impala, LeSabre, Olds 88, should work. (I left off the Pontiac, because the had several models in various years use the B-body chassis)
This B-body was made from 77-90 with the "squared off body style"
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Larry
79 Buick Regal (my first car)
78 Regal Sport coupe turbo 2bbl