We use a 3/8-24 grade 8 BOLTand two 1/4-20s on an 18 cubic inch 30 hp twin..The two small ones are just backups to keep the prop from slipping..The bolt goes through the prop and into the crank..
The style you have is held on by 6 flathead 10-24 screws..You could take out two opposite screws use two longer bolts thru the prop washer and spinner into the hub...NO more slip..Or..Take out all six screws and drill six holes in the prop....
One problem with this setup--you have to check on the bolts more often..When the prop gets compressed the bolts will shear off....