Another update.
Used a jack and some wood to prop the fuel cell in a rough area where I wanted it. I could move it around as needed while I took measurements. After that, I started laying out some 1/2" square tube for the skeleton.
I tac'd those into place and used the jack again to align the whole thing. I made some tabs for mounting, since I wanted the assembly to be removable and potentially allow for a different cell (alternate fuel possibly?) to be put in on 4 bolts. I also wanted my cell to sit as a certain height, and to naturally counter the rake of the truck.
I gusset the back side with the longer arms, and the whole thing could be set back into place. Then I drilled and tapped the tabs into the frame. The assembly has 4 bolts threaded right into the frame on 3/8-24 fine thread grade 8 hardware.
I also reused my old top strap with a few different bends in it. It hooks to the longitudinal strut I added (where the battery tray will start ...in the next paragrash
) and uses another 3/8 bolt, locking the fuel cell in place should gravity fail, or should I be upside down.
And after a bunch of grinding and some blending, I hit it with a few coats of prime and satin black.
At the same time as all this ^^^^, I was working on the battery tray also. Its a triangular skeleton with a flat stock floor plate welded into it for the hard mounting on the battery box.
I think its strong enough for me, so the battery should be ok.
I had to make a few small trims to the battery box plastic shell, mod the hardware, and narrow the battery hold down (was designed for larger battery than I run). The box and bracket dropped right in and sat firmly and squarely (triangularly? lol) on the base.
