Were the 60 and 70 bumper bolts stainless including the threads?

Thin stainless 'appearance' caps on steel carriage bolts.

so actually when the bolt is tight the treads that are showing would be shiny?

Stainless steel, steel with high nickel content, doesn't rust, correct?
To answer your question I don't think any American made production cars had stainless steel bolts on the bumpers

Yes but because they are NEW.

Chrysler did not use stainless steel bolts , they are as Barry described , a plated steel carriage bolt with a cap over the head of the bolt.

I see a lot of restoration on 70 super bee with the correct nut but the bumper bolt threads that protrudes are painted black which I thought would be incorrect. That was the reasoning behind the post

Actually Ford used some stainless steel bumper bolts on some 66-71 applications, as I bought a small quantity of NOS Ford Stainless bumper bolts a few tears back, ended up using them on a few Mopar restorations, last thing I like is rust stains oozing out of the bumper holes...screw correctness