Shoot, I just bought some a couple of weeks ago, but I can't remember the lengths. I know they were SAE, not metric, and I agree 5/16-18. And yes, there are two lengths. In fact, my old pump used different combination of lengths than my replacement pump. So even if I remembered the lengths, it might not work right for you.
Can't you just measure the pump housing?
Don't lose your fan mounting studs. Those will be impossible to find around town. The bolts will be easy to find.
Dodge did not build my Jeep.
1988 Wrangler,4.2L I-6, Carter BBD carb, 5 Speed Peugeot, Zone 2.5" lift + 1/2" booms, BFG 31 x 10.5, Bestop.