Garage-Girl

I’ve been on the hunt for a replacement box for my dads truck! He’s got an 08 Sierra ext cab 6.5’ box.. I haven’t found a box, east coast Canadian, finding anything not rusted out after 5 years is hard enough! I did however stumble across bed side panels.. but they’re for 99-06 .. all dimensions are the same other then the outside length which differs by 3/4 inch. Is this going to make a difference? I’ve been searching online, and keep getting conflicting answers! Some say 99-13 will work, some say 99-06 and 07-13 .. If it will, is there anything additional I’ll have to do? Will I need to modify anything? Any insight is appreciated!