Consult with Randy Schlitter confirmed that we can chase drill or ream the holes to attach the elevator horn since the load is in the other direction. Actually riveted the joining plate to both sides of the forward elevator spar first and then fit clecos in the two middle holes of the horn assembly, then prying these two clecos apart pulled the ribs in enough that using a straight reamer only removed minimal material and fit the rivets easily.
Riveted the first rib or two out from the middle of the elevator.