[rdmaster92]

The 68-69 Buick 400, 67-69 Buick 430, and 70-76 Buick 455 are essentially the same. According to Mitchell interchange, the 67-68 430, 69 Riviera with 430, and 70 Riv with 455 have a different oil pan, which would need to be swapped, but not in your case. I believe all you need to do in your case is bolt the motor mounts that are on your Wildcat's 430 to the 455, and it should bolt right in. You probably also need to retain the throttle cable and switches from the 430 setup.

You are right about the internal parts being harder to get for the 430. The valvetrain and heads in particular are quite different between '69 430 and '70 455. But the external dimensions are near identical.