Come on somebody has to know this.....
3/8'' 16tpi - i can give you the length too if you need but tomorrow.
look for a kit of stainless, i installed mine and then i found a stainless kit for $14.
and I had a broken stud too, by the firewall, unlucky me it never came out at all - drilled with a bunch of bits, got it almost in the center, manage to chase the thread on the first half and then the remainder of it came loose and i was able to unscrew it - but it took close to couple of hours and the worst part is that you cannot see there - center punched couple of times and checked with a mirror.