Nice lookin' truck and a cool thing to do for your dad!
Welcome to the board.
If you look at this pic there are 2 bolt holes on the front bottom so there must be a bracket. might find it on site linked to pic.
http://www.gmcpauls.com/pictures/swbbedsteps.jpg
or most likely it was riveted/welded to frame like the AD trucks and you'll have to hit the junkyard.
Post your question up in the 67-72 section and you'll get quicker response from guys that have/know that truck inside out. I'm an AD guy