How do I take this out?

Alaskan_TA said
The studs sheer off easily with age.

So if you have a tap & die set, chase the threads under the nuts to clean them.

Then spray the nuts & washers with a creeping oil of some type & let them soak a bit before trying to loosen them.

I can only imagine, 46 years of dirt, mud, oil, and who knows what else.