Adrian, for my battery relocation, I bought this box (not from this seller though), prepped it, primed it, painted it satin black.
http://www.ebay.com/itm/Optima-Welded-S ... bc&vxp=mtr
then drilled through the rear passenger's floorboard and bolted it down using grade 8 bolts and grade 8 nylon lock nuts. When I pushed the tray all the way against the driveshaft tunnel, two of the bolt holes went through the frame rail and the other two just through the floorpan. This left the fuel hard lines smack in the middle of the bolts (don't worry, I took measurements before I drilled lol) I used some grade washers to reinforce the non-frame rail side. Only thing that sucks is the bolts/nuts hang below the frame rail about 3/4 of an inch. Not much issue for me, but I'm not as low as you are. Also, get some high quality 0-gauge power wire, welding wire is usually cheaper than car audio wire, but still super flexible, heat and chemical resistant, super low resistance, etc. All you'd want in battery wire. Then you'll run it to the factory terminal on the starter solenoid. Place a fuse or circuit breaker (150amps is good) in the wire no more than 18 inches from the battery. Make sure that ground wire is no more than 18 inches in length.