SMitchell, Best to use a Chevy truck chassis to fit your '57 Chevy truck. A Mustang is much lighter than the truck and the commonly used M-II front end is really too light for a truck. Many have alignment problems because of the weight difference. Chevy to Chevy is the best and most likely to fit better. Changing the entire chassis is a very big job and should be left to a professional to be done right. Transplant the suspension from the later truck to the '57 if you have to use IFS. Everything else should bolt on. If you must change the chassis, measure wheelbase, tread, body mounts from both and see how close it will be and how much adaptation will need be done. Good luck, but I strongly advise against the chassis change.
