I ran across this metal cab dolly that lets you flip the cab up on its side and work on the bottom.
http://www.bumpandgrindgarage.com/1213226.html Looks like it would be good as long as your mounting holes were solid and you didn't need to work on them.
Unfortunately, is only for late-model cabs. Anyone seen this before? And just for a curious question, anyone know the cab mount bolt dimensions? I think you could build one of this with those dimensions, some 1" sq. tubing, castors, a saw and a welder.
P.S. Re-read this and it does include plans for Task Force trucks. I'd send the guy $30 bucks for his plans since it was his idea and would be easier to build.