The bracket on my right front fender was broken so I extracted a donor from a bone pile fender. Now ... how should I install it without mangling my fender?

Me bad! I used slot head screws with square nuts and lock washers.

ok ... I could do that. Guess I could fill the slots with rust to match the rest of my fender in that area. But was wondering how they riveted them ... hot or cold?

Cold! They had the equipment to simple squish the rivets. At the rates of production I don't think one would have time to use hot rivets. That would add to the production costs and slow the rate.

Enlist a helper to hold a heavy backup weight like a sledge hammer head against the outside ends of the rivets while you hammer the inside ends.