I will take a crack at this, however, be forewarned this is based on my '37 Chevy.
Ed, the choke, hand throttle, oil pressure line and water temp line go through the oblong grommet in the center which is held by the metal bracket.
Hole #1: I don't have these--it is not a GMC
Hole#2: One of the small ones is for a harness clip, placed inside. The others don't belong. Maybe you can tell which is original.
Hole#3: Yes, vacuum line to wipers.
Hole#4: I don't have that one at all.
Hole#5: For another harness clip, placed inside.
Hole#6: For the armored coil wire (Elektrolok). The smaller holes (6a) don't belong.
Hole#7: I don't have these.
Hole#8: I do have this, looks original, I don't know what it is for.
Hole#9: Yes.
Hole#10: I think that hole should be threaded and is one of the holes for the floorpan to be bolted to.
My truck did not have a voltage regulator, just a voltage cutout, which is mounted to the generator. The harness itself exits the cab forward to the horn and lights between the floor board where there is a sort of a cutout, and the cab just below your hole #5, and to the left in the picture from the driver side hood latch. If you look at your floorboard you will see what I mean.
The above observations are based on what little was left of the wiring on my truck, and examining another complete '37 Chevy. I hope this helps.
Kent
Last edited by Lightholder's Dad; 04/16/2020 12:13 AM.
