Dave I ran across your site a little while ago searching for custom tanks then came in only to see this post. Must be a sign.
I want to do a bumper fill also but I think the dimensions are going to need to change quite a bit for that. I'm waiting on my bumper to come in (Brothers is taking their sweet time) so I can get it bolted up and get some measurements. I think the fill neck will have to be at the bottom of the frame rail to get enough angle to fill the tank properly. I'm not real sure about that though since I don't have a way to fab it up yet.
Couple of questions though, can you bend (rolled, not square) the lower part of the tank instead of weld so there are only seams showing on the sides? (bends would be front and rear and sides welded in) The top could be welded all the way around since it won't be seen once the bed is on. Also, do you do anything to reinforce the sides where the mounting tabs are? Seems like a lot of weight on a small tab that could stress over time.
Thanks for your time and I'll send you some drawings and dimenstions as soon as I can.
MD