I knew there was a '60's version of a long-roof 2-door Mustang.
But considering the Fairmont Fox-body commonality with Mustang... what about a Fox shooting brake?
If a Mustang front end can fit on a Fairmont or LTD wagon...
What about fitting the back end of that wagon, onto a modified Notchback Mustang?
Imagine cutting the Mustang's body around the rear glass, and down through the rear wheel wells to the floor pan.
Then cut the Fairmont's back end off, using the rear wagon glass, rear hatch and bodywork aft of the rear wheels.
Cut the rear seat bulkhead out of the Mustang, and put in folding rear seats from the hatchback Mustang. Otherwise, if you start with a hatchback, graft the notch-back rear quarter windows and C-pillars onto the body-sides, as the notchback quarter glass suits the aft side wagon windows better.
With standard go-fast Mustang parts, it would be one fast shooting brake 3-door.
This is kind of along the same lines of my previous cut-and-shut mashup idea for a 3-door Subaru GC coupe with two side doors, with the 5-door's rear roof and hatch, and STI guts underneath.