Well George, have you checked out any axles at
www.RC4WD.com? They have lots of nice scale axles. Some plastic, some aluminum. I'm not sure what your budget is. Some get expensive, but are really nice.
A belt drive for the transmission is a good idea, but I'm afraid you'd have a hard time getting the gearing right. You will probably not have enough of a reduction and the model will be to fast, with no power. I'd look for a used transmission instead, that had gears. Some of the cheaper ones that come to mind are the Axial brand transmissions, which are used on Axial's rock crawlers. These are cheap, plentiful, and parts are readily available. You could also check on the Axial SCX-10 axles. These are more scale like axles. You might like them.
http://www.axialracing.com/
http://www.rc4wd.com/
Maybe you'd be best off buying a used Axial SCX-10, or one of their other trucks, because the whole drivetrain could be used, including parts of the suspension. Axle width might be the only hold up - if they aren't wide enough. I'm not sure of the specs, but you could probably find that info on their website.