A couple of issues here. Sometimes the placement of this part is crucial once the rudder cross arm is down as it affects whether the arm hits the boat frame on way through.
So set it all up and check that. This detail is finished a little differently to the boats downunder I think.
I would remove, fill, redrill and make sure there are no clearance issues (rivits have a low profile) with the bolts you use.
Bolts are not a prolem in themselves. As I have said bolts are a far better option in some places but a manufacturer will always go cheaper and faster.
Steel on steel and ali with ali is also a good rule of thumb. Don't cover or paint stailess as it will deteriorate if in an airtight situation.