[Espeefan]

Your welcome. The two biggest things that will help you figure out how wide to make the chassis is your body's width, and your axle width. Take into account width for leaf springs too, if you go that route. Most axles have a mounting pad for the leaf spings, and I would figure out the width of the springs you want to use, the width of the shackles, for the springs, and then the space between the shackles is what you'll want the two chassis rails to be, in width.

Get your axles first, body second, and then build the chassis to fit them both.