I have a set of those 452 heads with the tapered seat plugs. They also have the little dimple below the plugs. Mine were bought as bare castings over the counter about 1992/1993. They were then sent to a machine shop and had bronze valve guide "sleeves" installed and Mopar Performance valves (2.14" & 1.88"), comp cams valve springs, locks & retainers, and had "some" port work done. Nice thing about them was the smaller plugs helped the plug wire clear the 2" Hooker headers with out burning the wire ends. Not to mention none of the bolt holes were worn out, stripped out or rusted out.
The bad thing, I was in Indy in 1995 at the Mopar Nationals, and on the last time run sat, blew the center out of a Champion spark plug. No problem, we're surrounded by mopar people, right? Yeah right. Everyone I ask looked at me like I had two heads. Finally went to K-mart and found a set of Bosch Platinums for about $20.
(remeber, this was 1995!) Never went racing with out a spare set of plugs again. Those heads pick my car up by .2/sec over stock 906's.
Brian Dunnigan