Buy them long-ish. These will probably be #8 or #10 bolts which aren't so thick that they can't be snipped/cut to length after tightening the nut. I, too, would use a flat washer on the inside. Leave enough length to put the acorn nut on. A quick pass or two with a file would probably be sufficient to clean up the threads to get the acorn nut to engage. (Maybe a small stone wheel in a Dremel would ease the process.)
I'd probably sacrifice one bolt to confirm that my cutters (probably end cutters) were up to the task before I started wholesale rivet removal.
But, I'm good with rivets and haven't made the swap.
Quando omni flunkus, moritati