I went thru a bunch of '66 VIN tag pics - I found none that were welded,
the following appear old/original & rosette rivets (lowest VIN sequence found):

plant 1 - Lynch Rd - 102xxx
plant 2 - Hamtramck - 146xxx
plant 3 - Jefferson - 162xxx
plant 4 - Belvidere - 169xxx
plant 5 - Los Angeles - 101xxx, 515xxx
plant 6 - Newark - 220xxx
plant 7 - St. Louis - 144xxx
plant 9 - Windsor - 221xxx