If you want it to look original you can use hot rivets. If you want it to be structurally sound, I would use flange bolts.
http://www.imperialsupplies.com/item/0191620?service=/browse/Up until about 15 years ago large trucks were put together with flange bolts. They now use Huck bolts simply because they are quicker and cheaper. We do frame alterations all the time and use flange bolts. If a 5/8 flange bolt and flange nut will hold an 18 wheeler together, a 7/16 flange bolt will certainly be overkill for a pickup.
