Frank - Fittin' hoods on '47-'55 trucks is a miserable undertaking no matter how you slice it. Usually, when you get one side perfect, the other side is way out. Then when you get both sides perfect the cowl gap is out. This is made worse if you have disassembled the truck and done some body work on the fenders and grille area which will also affect fit.
There are four separate factors in hood fit that come into play in my experience (GMC): the firewall braces as mentioned above will open and close the fender gap full length but do nothing for areas that are not fitting, the washers/spacers under the lower radiator support will help to tweak some of the fit in the front of the fender to hood fit but a little goes a long way, the hinges and bolts will also but I've not heard of slotting the openings and just loosening the hinges generally will let you tweak it some, and the inner fender bolts must be loosened and then retightened when you have the line between the fender and hood the way you want it. It's a balancing act.
Remember that these trucks were often not tight from factory and a small tweak here and there on the truck will wreck havoc on hood fit and seams. Remember to loosen the fender to inner fender bolts too as they will have a little bit of slop to help open or close that gap some.
Jim Shekell is a guy who's been building custom AD trucks for over 30 years and he told me that he has trucks that he will only show with the hoods open because of this problem.
Know that you are far from alone in this problem.
