The simplest way is to get the front kit from CSRP for the set-up you currently have (manual or power). You can reuse your spindles from your 4 lug system so long as they are in good shape. Then find a 5 lug rear end from a 67 or 68 mustang. (the 65 & 66s are too narrow & I think 69 and up get too long) Other Rear ends may fit - you just have do your homework regarding all the important dimensions. The cheapest way is to find a donar car, but the labor in cleaning it all up and getting it to work increases. Finding an 8 inch, 5 Lug rear end shouldn't be too hard, but finding one in good shape can be tough and having one rebuilt will cost you about 500 more that having a late model 8.8 rebuilt, which would be stronger and come with disc brakes. I found a 9 Inch with tapered axle tubes that I was going to have rebuilt, but the bell proved to be rotted out so I went with the 8.8. - 8 inchers are also as much if not more than 9 inchers to have built once you find the core. A 9 inch core can be pretty pricey.
__________________ Ride On,
Noel
"I'll fix it or break it, Guaranteed!"
My Garage - 67 Mustang Coupe, 67 GTO, 67 Sprite, 72 Chevy PU, & Old Ford 8N. | 
