grats on the new PC,
http://web.archive.org/web/200410071426 ... rlarge.jpg
The tubular steel behind the rear wheels, another tubular steel mid chassis, and another tubular steel at the front of the bed, On your model, the rear tire's rear wheel well would connect to the tubular steel that then connects to the chassis, The front of the bed where ti drops down would connect to the front tubular steel, and your yet to be modeled gas tank area would be the middle tubular steel connection to the chassis. (IMHO)
The bed/cabin/dumper/etc would bolt on to the tubular steel.
You could drop the bed and leave wheel well cut outs,
or add some blocks between your current chassis and bed, not sure how leaving a visible gap between the chassis and the bed will affect the FPS and hit box regarding gameplay issues.
There were over a dozen variants, just pick one and make it work.