Because I want to run Corvette offset wheels under the fenders. It will save me $600 to buy a slightly used set of 17 X 9 / 17 X 11 Torque Thrust II wheels (my favorite) or I can save over $1,000 if I buy used 18 X 9.5 / 18 X 10.5 C5 Z06 Vette wheels (which look like the late model Shelby wheels).
I’ll keep the 9” solid axle rear end; I have 7.5” of backspacing to spring. If change to full size Lincoln rear end, I’ll have 8.5” backspacing to spring. The Vette wheels have 7.5”, 7.75” or 7.9” backspace depending on offset.
If I keep the Mustang front suspension, I can run wheel adaptors and fit the 18” rims over the A- Arms, or with shorter spindles, the 17” will work.
If I can find the labor cheap enough, however, I’ll swap in C4 Vette front suspension. I can get them for $600 rotor to rotor with 13” discs and remove the problem of shock tower stress cracks in road racing conditions. I can convert it for 1/3 the money it would cost to upgrade with Mustang compatible aftermarket parts. The Ford purists should not squirm too much at that because aftermarket parts are not Ford parts anyway, and most of my "all Ford" haters run Taiwan sheet metal on their “all Ford” Mustangs. At least mine will be all American and it will run circles around aftermarket Mustang road racing suspensions in the autocross.