If you rear locate the battery in the cab, make sure you use a Optima. It is a sealed battery & you wont get any fumes. You can mount it on it's side, upside down, how ever you want.
Quick & easy.
Custom,
Make a stand off bracket out of tubing that welds to the side of the frame & bends up to the bottom of the battery hole {in your body} & weld a plate to the top of the tubing to make the tray, secure the battery to the tray. Make sure you cut the hole in the body to have clearance when the body flexes.
If you make a box like 79coyotefrg says, make sure it has vents to let the fumes out.
I was thinking about the Optima for the in-cab location. I may still go this route.
I'm really not crazy about the bed because of the danger of stuff hitting it, even if it's in a sealed box.
The solid frame mount you speak of is interesting. The hole in the body is already taken care of
