This question came up on the other forum and I think the answer was that there were several differences.
- The wagon frame was boxed in more places than the regular sedan frame. [I am not sure how much different it is over the 9C1/9C6/WX3 frame in this respect.]
- The rear part of the frame is cross-braced at the horns. It is like it already has the Buick rear brace installed (but welded in).
- There are also other minor differences in body mounts.
Probably the differences in wagon frames account for the LT1 or more specifically the 4L60E transmission mounts.
- The wagon frame was boxed in more places than the regular sedan frame. [I am not sure how much different it is over the 9C1/9C6/WX3 frame in this respect.]
- The rear part of the frame is cross-braced at the horns. It is like it already has the Buick rear brace installed (but welded in).
- There are also other minor differences in body mounts.
Probably the differences in wagon frames account for the LT1 or more specifically the 4L60E transmission mounts.
