Diesel for a car? I'd start looking at a Mercedes. There are LOTS of them around ... and often the bodies rot away from a perfectly good drivetrain.
I knew a guy that loved Land Rovers but hated their drivetrain ... so he put a Mercedes 4-cyl diesel in his old Landrover. He drives it all over the country.
The guy lives in Connecticut and you might be able to find him on the Guns 'N Rovers bulletin board:
http://gunsandrovers.yuku.com/
It won't be much for acceleration, but you should be able to cruise economically if you really love the looks, etc ... of your mustang.
