Making dump truck

I just bought a Chevy Kodak single axle with a 3208 and 5 speed. I am wanting to put a 11' dump bed on it. It is not double framed, but would it need to be if I build my own subframe for the dump bed that goes up under the back of the cab and all the way to the back hinge? Does it need wood under the subframe or could I use 1/4" rubber belt? Gonna haul rock and dirt with it local, but don't want frame problems. Also thinking about swapping the 5 speed for a 10 or 13 speed. What would be involved in that? I'm familiar with the 3208 engine with a 5+2 but having just 5 gears might cut the power even more. I bought the truck cheap though. Thanks

Making dump truck