Finally the mounting holes...
Note that there is a larger hole in the actual 35" outer guard piece, then a smaller hole in the support pieces.
The larger hole is 1" diameter:
And note that the hole is NOT centered in the bracket's 5" height, rather it is 1-1/8" from the large hole edge to the top of the bracket, and 7/8" to the edge of the hole from the inner end of the tab:
The holes in the support piece are concentric (center on center), and hole diameter is 11/32":
When you mount the guard and bracket, you will also want some sort of support piece behind the bracket. The diagram the other member uploaded above shows two pieces of pipe-stock welded to a flat bracket - you drill the inner bed-box with the outer diameter of the pipe-stock (plus a 32-nd or 16-th). This way, when you put the clamping pressure from the antenna mount bolt onto the pipe, it will pinch the pipe to the outer fender, and use the large inner flat piece as additional torsional support, meaning if you grab the antenna base and pull it away from the truck, it will put even pressure on both the outer fender and the bed-box sheet metal. Also it you were to smack the antenna base into something, the inner bed box re-enforcement plate would keep the bottom mount bolts from tearing out.
Here is what they look like when mated (of course not on a truck):
Beyond that, I have a difference of opinion from how GM and the military did the cable pass-through. I don't like rubber gromets, they still wear out and let the hole cut the insulation of the wire and water pass inside (do we really need more opportunity for water to get inside and rust these trucks?). I prefer a short 4" jumper from the antenna base to the fender, and then an insulated bulkhead pass-through (like the below from Pasternack) with and O-ring and some RTV silicone adhesive.
Either way, ground-strap the heck out of your truck if you plan on doing any CB-ing or HAM-ing. Spot welds on older trucks aren't the best power grounds, but they are generally terrible RF grounds.
Best of luck,
-Tim