Was the 454 XJ bright yellow?

Frames? A yj has one, an XJ does not.
XJ is a unibody, the frame and the body are one piece.
What that means to a project like yours is, extensive reinforcing.
The yellow XJ has 3/16 plate lining most of the under side rails. Yup, it gains a lot of weight too.

Leave the axles full width.

The best rear J20 axle is the semi float 35 spline D60 with a 5 on 5.5 wheel pattern. The full float is a weak 30 spline d 60.

SD