I am in the process of refreshing a 70 240z build date of 6/70. It still has the original Toyo spare tire as a spare. How do you deem an old tire like this as serviceable?

Jim, for use as an emergency spare, it should be OK as long as it doesn't bleed off pressure too fast, and has no weather-checking that might be deep enough to expose the inner cords.

For a tire that has never been in use, there are two major age-related concerns. The first is that the fabric and steel inside the carcass has never been exposed to oxygen or moisture. So if there are no cracks in the sidewall rubber or in the bottom of the tread grooves that exposed the cords, you should be fine there.

The other concern is that over the years, all the volatile compounds will have leached out of the rubber. The grip and durability of the rubber becomes badly compromised. For use as a spare tire (seldom used, and at limited speeds and stress) it will probably be OK, but no way I'd want to use it as a normal-use tire. General rule of thumb for normal use would be 10 years old at the most.