I'm in the proccess of installing my LMC tank now. I've got the tank in and I'm getting ready to plumb it soon. I just welded a couple of square tubes across to each frame rail to form a box, clamped the tank to it, bent the straps to follow the contour of the tank, drilled and tapped the frame (which is double thickness and boxed back there) and bolted the straps up. I used stainless steel hardware so I didn't use the J-bolts. If you what, I can take a few pictures if you like, my truck is just a rolling chassis right now,
Scott