Stuff like this is what you figure out when building a truck. They are not set screws. Just eyeball the size, type of head and length that would be needed and buy several types at the store. They are cheap. Stainless or chrome screws. A GM part number would not help. This a is a basic skill you need to learn. Sizing hardware.
That particular control could not be easier. You have the countersink on the bottom so you know the head style (probably oval head) and the receiving female thread or tinnerman on the truck. At the top, you have the threads in the control casting. Take the control to Ace and dig around.
Parts like this control also need to be examined to be sure threads are still viable. If not, carefully chase them. You can't wait to find out there is a problem during install and then strip the threads.
Calipers and a thread gauge is nice to have. Soon you will just eyeball the size and know if it's coarse or fine. Back then, hardware was basic, not fancy. Standard sizes and not that many sizes.
The "bright" hardware/ "trim" screws are either sheet metal or SAE threads. The "countersunk" screws are usually oval head, a variant of flat head. They are stainless steel or chrome plated. Sometimes you will have flat head or pan head hardware in the small stuff. (screws). 99% of them will be "cross recessed" (Phillips). Some are clutch head and can still be purchased or switch to Phillips or slotted. Machine screw and sheet metal screw sizes will be #6, #8, #10, #12 ( .138, .164, .190, .216 ). The bolts will start at 1/4-20. Of course, there will be some odd balls. Ain't no metric.
