Hey, the tank looks good!
My frame has another cross member just be hind the axle and it sits at a slight angle. I think I have seen it on other leaf trucks as well but maby it was not always used. That is the other one I attached my straps to. As for the filler neck I first used a pice of 2" exhaust pipe and had it bent to go into the rear wheel well area. This would probebly be tough for you since it is lowerd. I later drilled a hole in the bed side staight out from the filler on the tank and used a stock stainless cap. I had a bit more room for routing because of the body lift. I am sure you will come up with something trick! I wanted to keep mine out of the bed because I used my truck alot and unloading it to fill up did not sound like fun..
