Some kits don’t have all the parts you need
I have enjoyed reading your Mechanic on Duty in Auto Restorer especially as it pertains to 1960s-era Pontiacs. I have just recently reassembled my 1967 GTO body to its chassis using a new set of body bushings and bolts from Ames. I also checked out the sets with OPG, Performance Years and a few other companies that carried vintage Pontiac parts. I was surprised to note that none of the companies offered the four bushings that do not require either bolts or washers, these being the two over the rear end cross-piece and directly in front of the rear wheels.
Consequently, I reused my old ones by cleaning them up and soaking them with silicone spray lubricant. They looked and felt OK.
I’ve mentioned this anomaly to the above companies but few seemed to be aware or knowledgeable about the four bushings I’ve described. Have you encountered this in your restorations? My assembly guide does mention three different types of bushings used on that year vehicle and in 1966.
My research has confirmed what you already know—the readily available body mount kits for your 1967 GTO do not include the four bushings you describe.
Your experience illustrates the importance of something that I continuously stress to restorers and would-be restorers: As you disassemble the car you’re restoring do not throw away any of the old parts, even the perishable parts that you intend to replace. You often will need the old parts so that you can put them right next to the new ones to make sure that the new ones are the correct size, shape and configuration. And as your situation illustrates, you may need to reuse some of the old parts that you assumed you would replace. And finally, in some instances you may be able to fabricate new parts yourself, have the new parts fabricated by someone else, or find an available part that is a close enough match to the one you need to function correctly, and for all of this it will be invaluable to have the old part on hand for comparison and reference.
When it comes to rubber parts like body mount cushions, weather seals and gaskets, some of the larger manufacturers are willing to help solve your problem even if they don’t make the exact item you’re looking for. Steele RubberProducts in Denver, North Carolina (steelerubber.com), may be able to help with the GTO body mount cushions you describe by matching your original ones with a functionally equivalent part that they do presently manufacture.