You might look at Bobs Chevy Trucks, www.bobschevytrucks.com and also LMC Trucks website. They both sell door hinge components and might have diagrams or pictures on their websites. For more parts sources for your truck checkout the Chevrolet page on https://www.vintagepickuptruck.com
Check with Don’s Auto Parts, www.donsautoparts.com , he has a lot of the odd and hard to find parts, NOS and used. I am not sure if he has everything listed on his website so you might want to call him, his number is on the site.
Try Tail Light King, www.taillightking.com also try Don’s Auto Parts, www.donsautoparts.com . Both have used and NOS lights, lenses, marker lamps etc... might have what you need.
Checkout the Studebaker page and the Steering page on https://www.vintagepartsource.com , parts suppliers listed that might have the parts you need and also some rebuilders listed for steering components.
Try the Pontiac page on https://www.vintagepartsource.com , there are several suppliers listed that have some hard to find and obsolete parts, NOS and used.
Try https://www.vintagepartsource.com there are pages dedicated to suppliers for those car brands you mentioned, Studebaker page, Hudson page and the AMC / Nash page.
Try Olsons Gaskets he has a large stock of gaskets and can also make obsolete gaskets. Also check the Cadillac page on https://www.vintagepartsource.com there maybe some other suppliers with those gaskets.
Check the Oldsmobile page on https://www.vintagepartsource.com several suppliers listed on there that have a lot of NOS parts, one of them may have what you need.