Yes they were aluminum rivets and I believe 1/8 mandrel. The pieces really did come together nice and tightly, on a couple occasions, I had to redo a couple and clamp the two pieces with very small vise grips with some of that cured rubberized coating on the jaws. Funny thing, I set out thinking if it didn't work out to my satisfaction, I could always use the stainless hardware and fasten it with small bolts,lockwashers and nuts but this ended up a cleaner install than all that. Disassembly was really easy by drilling out the rivets.

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration