I know this question is old, but I’d like to share my experience.
My 07 Fit’s battery died last week at a very inconvenient time. No local parts stores had the 151r battery, so with little time to spare I took a chance and bought a store brand 51r that was about 2" wider. I removed the stock battery’s tray and cover to fit in the larger 51r, tightened the bracket to hold the battery in place, and it fits/works wonderfully. It took me literally 5 minutes with a standard socket wrench and cost half the price of a 151r!