Pulled all 8 plugs yesterday, and noticed the ground electrode on both #3 and #6 had collapsed on top of the center electrode (i.e., there was no more gap). Car seemed to run just fine before I pulled them.

First time I have seen this. I've had several sets of plugs on this engine.

Plugs have about 500 miles on them, they were all nice and tan. What gives?

I'm also running an MSD-6AL.

Two possiblities either they were closed when they were installed or something has been bouncing around inside the combustion chambers... And BTW if something were bouncing around cyl 3 & 6 share the same plane on a dual plane intake so it wouldn't suprise me to have both cylinders effected..

Wrong (too long) sparkplugs?

Brought the plugs down to Blair's Speed Shop today... plugs were too long. I looked at the other plugs - all were undamaged, so it seems that the ground electrode on the damaged plugs were rotated in such a way in the head, that the piston tapped the gap closed (probably on the first crank).

I got a set of Autolites, which were way shorter. We'll see how it goes.