No apology needed and I feel your pain, just not sure how to help as I don't have the experience others on here definitely do.
The two brackets that bolt to the body appear to use "optional" washers. In the assembly drawing, they are labeled "AS REQ'D".
Others need to chime in but I assume the "inner washers" between bracket and body are used as shims but there is no guideline nor measurement shown on the drawing to determine if you need them.
The washers under the bolts are "3", with a PN neither you nor I could find with an online search.
The washers between the bolts and the body (shims?) are "20" with what I read as PN 3760886.
In this case, AMK DOES have a listing:
https://www.amkproducts.com/search-result/?ser=3760886, which shows them as 1.88 diameter with a 0.47" ID. This tells me the bolts should be 7/16". Are your bolts 7/16"?
You can guess better than I can, but it seems your brackets were shimmed with these large washers.
Do you see "witness marks" on the brackets or body from these washers?
As to the bolts and washers, AMK sells a kit, G-7466 for the bracket to body:
If you click on the "PIC" contents, I find bolts with their PN B-10182 that are 7/16-14 x 1.5 with pointed tips. They don't have the same PN as the Pontiac assembly drawing but they are likely equivalent and will hopefully confirm the size of the bolts you have.
Similarly, the washers under the bolt heads are in the AMK kit as their B-11286. Again, not the same Pontiac PN but should help confirm what you have.
Good luck and I hope someone with experience chimes in on how to adjust the rear bumper. I have a rear bumper waiting to install and have seen a few that look wonky. I'd love to see a procedure for aligning these.
Mike