It might be too late, but i found riveting the angle stiffners was easier to do before the rudder stops were installed.
Can't comment if this is ok or not, but I will say we had no trouble getting to these. Do you have a small tungsten bucking bar in your inventory? This guy is work 10x it's price. It has an angle on one side that allows you to get into small spots like this.
https://www.cleavelandtool.com/collections/riveting-and-bucking/products/tungsten-mini-bucking-bar
Okay to substitute MK-319-BS blind rivets for the 3-3.5s here? Having a ridiculous time trying to buck these.