641,
I don't have my fenders currently mounted but can you center the fenders by using the bedside bolt holes? That would get you reasonably close. And if needed you could lower or raise your bed blocks as needed. I replaced my entire bed(Mar-K) and am using fiberglass rear fenders and running boards. Aprons are metal. I mocked mine up with C-clamps and spring clamps then removed for paint. Hope this helps. Have fun and good luck.
Dan
