First off, it's your truck do what you want to it.
Anyhow, The 3800 truck frame is flat on top, there's no rear axle arch. Once you figure out where you want your axle, you could just whack the end of the frame off.
If you're wanting a shorter, lower truck, the 3800 probably isn't the best option to start with. You would need to c notch the frame in the rear. As far as the front goes, the 3800 is quite a bit different than the 3100 or 3600 trucks, so lowering may be an issue. A new chassis, or find someone parting out a 3100 may be the way to go.