This is a new forum, so please bear with us -- somebody should be along soon to give you a good answer. I, on the other hand, will give you a half-baked answer
In my '49, I have the seat all the way back and it's against the back of the cab as it is. A different seat may not give you any more room than what you get now (the gas tank is low enough that the top of the seat clears it.
Also, something to think about -- later bench seats have their own supports -- so if you take out the riser (involves cutting the truck which I am not personally in favor of), you can put just about anything that fits width-wise.
Hope this info helps.