I think I can get a 31 to fit in my gen 4. Comparing the Interstate 31 I have to my 27's, the 31 is roughly 5/8 longer. Its also a little taller but that won't be a factor. It's going to come down to wether my battery tray will accommodate the extra length, and I may have to change the cables up. The posts on my truck batteries are at the edge of the battery, and the factory cables terminals are essentially a 90 so that the cables run down along side the battery. Where as the 31 posts are closer to center. If it physically fits without having to modify the battery tray I'll rework the cables as needed. I think it would make a big difference during the cold winter months, not to mention would be taxing them substantially less compared to the 27's.