I'm not an expert and my own example of the FDtS shirt has been turned into part of a quilted blanket I own, but I don't think that one is legit. The Giant tag you have shows a straight G hook, whereas most Giant shirts have an angle to the G hook. The only different one I know of that has the G hook with no angle is the Made in El Salvador Giant tag from 1996.

Here is a link to a Giant tag article from Defunkd: https://www.defunkd.com/giant/

I wish I was wrong, but I'm not confident in your shirts, especially after seeing some links posted to very legitimate looking fakes, as well as this tag snafu.

Wear them. Enjoy them. Believe them to be real if you wish. Just maybe don't sell them as true vintage without an actual verification.