And now, the rest of the story...
Let me preface this by saying I was running my timing locked-out at 35* for years prior to wanting to add vacuum advance to my RB.
I also have overdrive and 4.10's on 28" tall 325/50x15's. I though that vacuum advance would enhance my operating envelope, however it resulted in a broken piston ring land and 2 blown head gaskets with erosion damage to several aluminum head surfaces.
What happened? Detonation. The low rpm on the freeway combined with this additional advance from the vacuum advance lead to significant engine damage. There was no detectable noise, knock or other indication it was even occurring, but when I got home, the white smoke from my exhaust told the tale.