I think it'll work out for you. Since there's an extra metal strip, that should make up the difference. The metal strips overlap the wood a bit on each side so they don't add the full 1-1/2" of their width.

Here's some good info Bed Wood dimensions

BTW, the change from 9 boards to 8 boards happened mid-51.

If you got your parts from Mar-K, they should fit fine.

Kevin
1951 Chevy 3100 work truck
Follow this saga in Project Journal
Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.