Go here
www.cachassisworks.com and click on how to. Then click on "Subframe & Suspension". Just follow the pictorial on the 65 Mustang. You are basically removing the back half of the car, re-installing larger wheel tubs (tubbing) going to a 4 link or ladder bar suspension set-up, and with a shortened rear end, you can now install about any size tire you may think you need. Or you can do what they call a mini tub....
Hope that helps,
Ron