Reading spark plugs

I'm using Autolite 3924 plugs in my aluminum headed 454, Holley 780 carb, appx 10:1 compression. Yesterday, I put 2 new plugs in #1&2 cylinders, wound it out to about 60 mph for awhile, then coasted to a stop, shutting the engine off as soon as I could. Replaced those two new plugs with the ones I took out, then headed home. Back home, I cut the threaded base off one of the new plugs and found that the insulator was mostly white with a slightly tan band about 1/8" wide at the bottom. I was expecting to see some black so I could get an idea of where to go with a change in jets, but maybe more running time was needed to see that?
The jets that are in the primaries now are 72's any ideas where to go from here?
Thanks,
Rich