Recently finished removing and cleaning the tank on my '41. Starting with the seat back, it is removed by lifting straight up. There are large flat hooks on the top of the seat back that slip through slots in a reinforcing strip on the cab body. The bottom of the seat back is held by tabs that insert into slots in the seat bottom. Once the seat back is removed the seat bottom simply lifts out. You will see that the rather clever seat adjustment is provided by 2 pins on the rails over the gas tank and matching holes in the seat bottom frame. When you lift the front edge of the seat bottom you can slide forward or back to position different holes over the fixed pins. The seat back simply follows the seat bottom because its tabs are in the seat bottom slots and its top flat hooks at the top hinge in their slots.
Back to the gas tank, there are 4 seat bottom support rails bolted to the gas tank surround. Remove these, remembering that the outer ones only have the seat positioning pins
The gas tank is held in place by two straps bolted on each end through the floor of the cab. As previously mentioned these are easily broken. Clean the threads with a wire brush and a lot of oil before trying the nuts. There is very little holding the top of these, only the flat strap. One trick is to hold the long threads portion with a pair of vice grips while loosening the nut with a box end wrench . Once the nut is backed off a ways you can re-grip the threads under the nut. The threads can be repaired with a thread chasser or die.
