I'm glad that I could give some helpful info. Awesome find on the bed liner, free is the best price. I put them on my trailer too a lot cheaper than the store bought ones and they do the same job. Don't forget that when you screw them down to only use treated deck screws about 3/4 of in inch max, they will last a lot longer.
P.S. the front has the ridges too and can be cut up for short pieces of runner or grips for the shed. Post some pix's when done, good luck.