You can mount the entire 1954 Chevy front clip on your 1951 GMC.

Where the fenders bolt to the cab, is the same on 1947-55 GMC and Chevy cabs.

You can't mix fenders and grills.
You have to use the entire front clip from ether a:
1947-53 Chevy truck or a
1954-55 1st series truck

Definition of front clip:
Two front fenders
Fender skirts and supports
Front grill and radiator supports

Also you can mount the entire front clip from a GMC truck onto a Chevy truck.

If you do the clip swap the only way to tell if it is a Chevy or GMC is to look at the interior dash.