Here is a decent looking NORS master cylinder for a good price.
http://www.ebay.com/itm/1938-1946-Chevrolet-Truck-1941-1946-GMC-Truck-Brake-Master-Cylinder-Assy-NOS-/281827895202?hash=item419e4067a2:g:0BAAAOSwd0BV5lDw&vxp=mtr
If the brakes are restored correctly, the single reservoir is adequate for the design parameters of the truck. I hope you are planning on replacing all of the brake lines and hoses as well as rebuilding/replacing the wheel cylinders. Inactivity from sitting long periods is hard on the hydraulic brake components.
