Here is a wood mockup of the 1972 Thunder Jet, it shows the dipped front cross member to facilitate a smooth sweep for the tuners.
The 71 chassis right beside the 72 chassis. You can really see the different in where the driveshaft is positioned, how the engine mounts, and general improvements to the bulk head area.
Here is a really rare shot of Jim testing his 71 Thunder Jet in Grand Mesa Colorado. If you look closely, you'll see two spindle mounts on each side of the front bulkhead. It was used for testing different ski stances.
Here's the 71 Thunder Jet.
Here's the 72 Thunder Jet.
This is actually one of the prototypes for 72. It looks pretty sharp, but I think they make the right call with the current 72 glass.