The stock master cylinder has a 7/8 inch piston, and the F-350 master cylinder has a 1-1/8 inch piston. The mounting bolts on the Ford Ranger's booster are actually spaced 1/8 inch wider than the F-350, so you'll have to use a carbide bit in an electric drill to widen the mounting hole a little on the F-350 master cylinder.

Connecting / Adapting The Brake Lines:

One of the important things to note is that the rear reservoir on the F-350 master cylinder goes to the rear brakes, and the front reservoir goes to the front brakes. It's opposite on the Ranger. Make sure when you connect your brake lines to the new master cylinder that you're connecting the correct line to the correct port. You'll likely need an adapter so that your brake line fittings will connect to the F-350 master cylinder. The 1985 Ford F-350 master cylinder is listed as having:

• Port 1 Thread Size (Forward) (In): 7/16-24 Inch

• Port 2 Thread Size (In): 3/8-24 Inch

A 1988 Ranger / 1996 Ranger (checked random year range span):

• Port 1 Thread Size (Forward) (In): 9/16-18 Inch

• Port 2 Thread Size (In): 7/16-24 Inch

The Ranger Port 2 needs to go to the F-350 Port 1 which should both be 7/16-24 inch The Ranger Port 1 is 9/16-18 inch and has to go to the F-350 Port 2 which is 3/8-24 inch. Therefore you need an adapter to take the 9/16-18 inch male thread, and install it in to a 3/8-24 inch female thread. Dorman 490-543.1 (3/16 In. Female to 9/16-18 Male Inverted Flare Dual Brake Line Adapter) should do the job. 

It's also important to note that all the master cylinder does is push fluid. The proportioning valve (on the frame) is what determines how much fluid goes where. Also, if you do this swap and your brakes feel squishy, it's likely because you have the wrong brake lines hooked to the wrong port. If you look at the F-350 master cylinder, you can see that the rear port is much larger than the front port.

Links:

Ford F-350 Master Cylinder Swap Discussion #1 Ford F-350 Master Cylinder Swap Discussion #2