OK, it's time to reveal my "secret". Use Scott Drake 351603-S tubular rivets. They are supposed to be for 64-66 Vent Window Seals but a quick-rough measurement (with a dial caliper) shows the head size to be about the same size as the 64-65 grill end rivets and the rivets used for the two brackets for the grill bars, 8 required, and enough length, 0.200 inch to hold the grill ends to the grill, 0.100 inch (I just measured it with the same dial calipers). The only issue is the body diameter. A quick-rough measurement comes up with about (I'm measuring thru the plastic wrapper, different calipers - "indoor" ones, the other one is a "garage" caliper. You don't have two?) 0.120 inch dia.
The ones I have came from NPD, 21450-1AK, a kit of 10 IN BLACK OXIDE finish. You need a special tool to install.
Jim