have you looked thru the 30's trucks in the gallary?
the bottom front of the fender bolts to the back edge of the board, doesn't sit on top of it, and I think it has a section there that's shaped to match the end of the board
I think usually the glas fenders are not a real good fit and have to be fiddled - should be strong enuff to take normal use, as the running board should be well attached otherwise and has some strut supports, doesn't it? fenders should have some rods for the bottom skirts to the frame too, at least they do on later trucks
Bill