I'd advise to measure the box, buy one to fit and just get all the cranking amps you can afford. My truck came with a small battery and like an idiot when I went into NAPA to get a new one, I just had them give me one like I took in, since it had lasted for about 7 years. But I also have to shove 1X2's down all four sides to keep it from sliding around the box.
And the PO let me keep his nicely tooled leather belt that straps around the battery and box to keep it from hopping out. He was pretty proud of it, too. I've just always gotten the highest cranking amps as possible because going out to start your truck at 4am on a freezing winter morning before work is not the time to find out you should have gotten a better one.
