Mounting the Step Mast on a MFG 19

[CCGunning]

Hi I bought a 1978 MFG 19 trailer sailor that I'm refurbishing. The mounting plate ( step mast? ) was screwed on with four mismatched screws including iron. There is a mounting block for the mast step of around 3 inched thick. Am I okay with just filling the holes with epoxy and securing it with 1/4in stainless screws? I would prefer going all the way through but cannot find stainless ( i'm saltwater ) big enough to get through the block and the wooden rib that is underneath the mast. Thanks for any thoughts on this. I'm retired US Navy, first sailboat.