12.5mm is almost 1/2", that's a bit of a reduction in thread engagement, I would either run extended studs, or get a spacer with built in studs.
On the subject of extended studs, I've heard of people running Tundra studs and ARP studs. The ARP overall length is 2.6", does anyone have the measurements of the Tundra studs and the factory spyder studs?
Edit: I couldn't find info on OEM studs, but Dorman lists their spyder replacement stud,
found here to be 40mm length. The Tundra studs they list as 44.5 mm length,
found here. The Tundra studs are also .03mm smaller diameter on the knurled area than the spyder.