A cutoff wheel on an angle grinder should be able to make the longer stud fit, as long as there's enough thread length to let the nut tighten. if not, a 1/2-20 threading die should fix the problem. Making anything shorter isn't much of a problem- - - - -stretching, on the other hand- - - - - - -LOL! You could also cut a piece of 1/2-13 all thread to length and use a 13 nut instead of a 20. The starter won't care.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!