Need help on rear end length and install.

[OldJimmy57]

First off since i talked to most of you on this board my plans have now changed. I have a 57 GMC Big Window long bed that I was gonna cut up and mod. But I figured since the truck had been in the family since 1956 that it wouldnt be right for me to go cutting it up. So instead i bought a 1955 Swb stepside chevy with a big back glass to start with. Now here are my questions. I want to put a Heidt's 4 link parallel bar kit in the rear to get the rear end down on the ground and to ride better. I will also be installing a Ford 9 inch rear end in this truck. Now I need to know what offset on the tires i need to go and also what length I need to tell the shop to cut the rear end to. I will tell you guys what I would like to run. I want some 15x12 or 15x14 rims in the rear.