Progress...

Welcome to the backup plan. I can't see any way I'm going to get all the metalwork done on the Mustang in time for Hallet in the fall. Not to mention that the new rear axle requires a new drive shaft, and it wouldn't make sense to build a new drive shaft for a temporary transmission. Oh, and 14" performance tire are almost impossible to find and stupid expensive, especially given the temporary rims.
So now what?
I have a complete but non running XJS. Its only significant rust issue are the floor pans. I now have a bead roller.
It needs new bushings all around, but poly bushing kits are available.
There are a selection of performance tires for the 15" wheels.
If the engine and transmission are in decent shape it could be in running order faster and cheaper than the Mustang.
If. 
http://xj-sprehe.blogspot.com/