In looking thru some old threads, it seems that M8-1.25x35mm is the recommended size if you don't want to use studs again. I've read some that say that is the size of the bolts that came with headers. Header flanges are usually thinner than manifolds.
I'm sitting here measuring a stud and nut that all came out together, and it's closer to 40mm from the back of (whats left of) the nut to the end of the stud. I would think more threads in the hole is better, but how long is too long? Looks like 40mm would be about right, especially with a flat washer.

Jeff