3/8"-24 (fine thread)

The bolts don't need to be more than 3/4" long or so. The shorter the better; you don't want them bottoming out in the plunger hole and letting the plunger slide down in the bore. We don't all mean to sound maniacal about the o-ring deal, but what a PITA it is to have to replace them after a rebuild.
The plunger bores are like razors on the inside of the case, and they'll destroy the o-rings every-single-time.