John,
Below are a couple of pictures that confirm (to me....) that the differential cover bolts are all the same length. The first one is from the fastener preparation that I did on my restoration project and shows all bolts as 5/16" diameter by 1" long.
The second picture is of two bolts that i just removed from the differential on my parts car. 65 Skylark, 10-bolt non-posi, but I'm sure that doesn't make any difference. The bolt on the right in the picture is the top one that secures the bracket for the brake distribution block. You can see that it is the same length as the one on the left, that was located next to the top bolt. 5/16" diameter by about 15/16" long (probably spec'ed as a 1" length bolt).
There don't appear to be any head markings on the bolts that I took out of my restoration. But one of the bolts that I just removed from the parts car has "TR" raised in the center of the head.
Chuck