I wonder if anyone has tried a Toronado/Eldorado type conversion to the 66-70 Riviera? The special transmission ought to bolt right up to a 430 Buick engine. The other main clearance issue would be the oil pan, but thta can be modified. (I owned and restored a 66 Toronado and even Olds had to modify the oil pan for clearance)
I've always wanted to build an all wheel drive Toronado/Eldorado/Riviera but that gets into engineering over my head. It's been several years since I did my Toronado so I totally forget if the front suspension is in any way compatible with a Riviera (in terms of geometry and placement) but I remember that these cars were all designed and built on the same platform.