Need help with valve adjustment on a '77 3.0

[MasonAutoX]

The rear of the car is currently in the air on jack stands and the lower valve covers are removed. I ran into an issue with the valve covers, as when i tried getting the nuts off they occasionally pulled the studs out with them. I have tried reinstalling the studs but they only screw back in about half the way compared tot he other studs. And I'm about to go and adjust the lower valves, but a 13 seems too loose on the retaining nut and a 12 wont fit. I don't want to try and risk rounding one of these, so I'd rather ask for ideas on here first as well as how I'm supposed to put the extracted studs back in all the way

[timmy2]

Double nut the studs, tighten them to each other with 2 wrenches so they lock and then install the studs using the top locked nut and a wrench.
Crack the two nuts apart and spin them off.

[al lkosmal]

I know this is blasphemy, but try a 1/2"

13mm is .512
1/2" is .500
12mm is .472