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I'd suggest a slightly modified version of the slot method: use a 3" cutoff wheel (or a worn down 4" if you don't have a 3" air cutter) Cut a slot like JF's son, stopping before hitting the sheet metal. Now cut another slot perpendicular to the first one to form an X. The quarter pieces of head will pop off with just a tap of a chisel. I have a Craftsman all steel wood chisel that I use, looks like a cross between a cold chisel and a wood chisel in that it has a 1" wide blade ~ 3/16" thick that is flat on one side all the way to the edge and tapered on the other side. Lay the flat side against the sheet metal and hammer away. If you can't find one like mine, you could make one from an old flat file. Files are made from high quality tool steel, and you can often pick up a handfull of them at the flea market for a buck or two. Learning how to anneal and temper tool steel is easy, check knife making sites for instructions, they often make blades from old files.
Allgood advice from people who have done it. I used weld studs because I think that they looked better. I might add that one of the reasons to remove studs is if you use a '57 to'72 bed side on a '53-56 is that the non running board cabs have the fender studs over 1" higher, and so you have to remove all the studs, weld or braze up the holes, redrill the pattern, then put in new studs.