It was tough trying to decide how to go with the rivets. After doing some searching, I couldn't find anyone with the right rivets or tools. I just kept going back and forth with what we were going to do for a couple months. Then I finally found Jim Dix (http://www.bigflatsrivet.com/), he has all the hardware necessary to do the job perfectly.
I used a regular air chisel with the attachment I bought from Jim. The air chisel is a standard tool, you can get one at Home Depot. You can see it in the picture on my site, it's the smaller tool in the middle. Doing this properly really requires two people.
First for the removal, the way we did it, was to take a very sharp chisel & basically knock off the parts of the rivets on the inside of the bed. Some would come off with one hit and hopefully and panel damage would be internal and could be hidden (hopefully).
Then for the replacement, I lowered the air pressure in my compressor(not sure how necessary that was, just made me feel better, some of the early tests were a mess). Then with someone holding the larger bucking bar against the head of the rivet - flat against the outside body, I would hit the inside end of the rivet with the air tool a few times to create a rounded head on the inside. That's really all there was to it. What scared us for months was totally done in 30 minutes and turned out perfectly.
I used 3/16” Round Head Aluminum Rivets in two lengths, you can see pictures here.
The shorter rivets are used most, but for the rear where you have multiple layers of galv trim, I used the longer rivets & cut them to size.
I hope this helps, let me know if you have any other questions & good luck!
On a side note, I recently was at the Land Rover factory in Solihull for their "Home of the Legend" tour. The restored Series III they had on display actually used pop-rivets for all the trim pieces, it looked very strange in comparison.
We also used standard pop rivets where needed(ie. around the gas filler), and I ordered a pneumatic air rivet gun off eBay, this was probably one of the most useful tools I purchased. Riveting by hand gets old after the first 5 or 10, these speed it up incredibly. But be careful, the first ding I made in the brand new paint was from the recoil.