The real question would be how strong is strong?

I'd not fish these without testing them. In any normal test, the thinnest portion should break first...if that breaks at a higher test than the tippet you are going to use, then age deterioration, even it it exists here, doesn't matter.

But, if the testing shows a failure above the tippet end, then I'd not use the leader at all, regardless of where it broke.

And with leaders this old, I'd not assume all were equal. Since you didn't tie them, you can't be sure they were tied at the same time or were from the same age materials. I'd test each one separately.

Or you could just give each one a good tug, call it okay, and fish with it...if it breaks, what have you really lost?

Buddy