jj, It sounds like we're starting the discussion at step two; were you not able to get the remainder of the stud out of the fork leg? Did you drill it out already?
If you have to repair the thread in the fork leg, going with a different thread density will cross-thread and weaken the thread. Since it's only one of four studs doing the job, that could be acceptable, but tapping to the same thread density would be better.
You can get a 5/16" X 26 tpi tap from British Tools and Fasteners. Not a bad thing to have on hand anyway. I have taps and dies for 1/4" X 26 and 5/16" X 26 and have used them on multiple occasions.
BTW, British Tools and Fasteners can also supply a correct replacement stud and nut.