This rivit talk is interesting.At one point in my life I was a Bus mechanic. It was commonplace for spring hangers to become damaged from loose spring pins. We would remove the rivited forged hangers and the replacement hanger was bolted back in place.The replacement came with flanged bolts as an factory approved replacement assembly.We also replaced hangers using conventional grade 5 bolts. Never had a problem.My 37 chevy truck frame was bent from overloading when I got it.I removed the rivits and rear crossmenber and springhangers to enable me to straighten the one frame rail. I remember drilling the rivit holes to get a proper fit on the bolts.Assembled it with grade 5 bolts.5 years later its still fine.Your frame was rivited at a factory of the largest vehicle maker in the world, on a jig, using engineer designed multi ton rivit presses.Do you think its possible to get this done the same way as when the truck was new? The rivits may also have been cold forged into place.