Overall length of the Stock stud really doesn't help much. Harbor Freight has a $15 dial caliper when you open the jaws a thin probe extends from the bottom for measuring depth. Pull one of the studs and place the bottom end of the caliper on top of the hole and extend the probe to the bottom and read the distance. Subtract a bit for the washer and lock washer and a tiny bit to make up for the threads not going all the way to the bottom of the hole. Doing it this way takes into account the head and base gaskets. Try to have 1/2" to 5/8" of the bolt into the block at least.
If your going to do this I think it would be a good idea to tap the hole. Quite often they are not tapped to the bottom and there are some, like mine, that were slightly undersized and a Sick Bike Parts head stud would not screw in and, no, they wern't 6mm. They were supposedly 8mm but not. I guess Chinese spec tolerances are pretty large. A good 40 piece Metric tap set can be found at Harbor freight for $12.
If the bolt you find is just a bit too long grind it or cut to fit and use Blue Locktite. Red you'd have to use a torch on the block next to the bolt to even re-torque the bolt from streching or remove.
Or just pull a stud, screw the acorn nut on as tight as you can get it and measure the exposed stud. It would still be a good idea to tap the holes, though.
