[Zeke3]

The picture shows the crank nose, and inner hub and the accessory pulley. I assume the slot in the inner hub is for the keyway and the inner hub is installed such that it is pressing on the oil slinger and crankshaft gear.

I would fill most of the gap between the nose of the crankshaft and the inner hub with a spacer, leaving the inner hub slightly higher than the spacer. Then use a thick washer, no larger than the outside diameter of the inner hub so it does not press on the accessory pulley, as Ronnie suggested. Also, a grade 5 capscrew will be perfectly adequate at 65 ft-lb torque.

If that thing still falls out, I would safety wire it to one of the other bolts on the accessory pulley.