Getting extra cab space is the only reason I could understand wanting to move the fuel tank, but you'd want to put it between the frame rails, with enough "buffer" between the front and rear of the vehicle to prevent a rear collision from causing tank rupture.. which is probably why GM engineers put the fuel tank in the cab in the first place.
Only time fumes would be an issue in the cab is if you spilled gas while pumping and it ran down the filler neck into the cab, or if you've got a leaking fuel tank.
