i would base my measurement off of a few things 1 being wheel width and offset to determine how wide the rear end will be and also you have to first decide how you want the tire to sit, do you want tucked in and slammed completely or do yo wanna have one those where the bed of the truck is a bit taller with the tire flush to the side of the bed. also your suspension will determine a lot also will you be running coil overs with 4 link or ladder bar leaf springs etc... there is alot of variables here
I just finished building a 9 inch for a customer's S10 about 2 weeks ago. The very first thing you have to do is get the wheels and tires that you are going to use and get them under the truck in the position that you want them in. Then measure between the mounting surfaces of the wheels so that you will know how wide the rear end needs to be. Then you can start on the rear end.
Narrowing a rear end requires some special tools. You must have a housing fixture to hold everything in line with the differential bearing bore. You also need very good welding skils.
The S10 has about 12 inches of room between the bed side and frame rail. To get a 14 inch wide tire under it you will have to move the frame rails in about 4 or 5 inches at least.
I narrowed a 9" for my "89" S10 for 16" wide Hossier slicks, the frame ended up being around 24" wide, moving each side in about 8-1/2". You have to have the wheels and tires first to see how narrow you have to go. You will also have to tub the bed depending on how much backset your wheels have. Otherwise they will be sticking out past the bedside.