They tightened up just fine. There was a fair amount of friction between; the mating surface of the domed head, the shank thru the ball joint and the a arm seemed to hold the bolt and provide enough resistance to allow sufficient torque. Additionally you can use the strike method (Slugging wrench) to make darn sure they are tight.
These bolts were a fair compromise. I wonder how many tons it takes to clinch the real deal rivet?