Here is a place I get most of my bolts from.
Link to stainless:
https://www.fastenal.com/product/fas...%2FThread+Size
As i don't have a 60-66 truck on the place ,
I cannot directly answer your question.
I can share
For The 3/8 bolt,
You could go HD or L oewes and get a sample bolt to try in the hood and fenders.
3/8 bolt will either be 16 threads per inch if coarse or 24 TPI if fine thread.
3/8x16 TPI or 3/8x24 TPI
5/16 bolts will either be 18 TPI if coarse and 24 TPI if fine thread.
5/16x18 TPI or 5/16x24 TPI
Hope this helps
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