RV 452 Heads....spark Plugs

I have an RV 440 with 452 Casting Heads that are original to the motor. IE...RV Heads.

I'm trying to determine a spark plug recommendation. I purchased ACCEL 437S Shorty plugs as they were recommended by other members for header clearance.

They DON'T fit. See pics. The bottoms side of the spark plug hole has a nipple that sticks out that interferes with the plugs. I had some old autolite plugs sitting around that I tried. They fit right in, but the "reach" is way longer than the Accel's and I'm just not sure if that's right.

Also, it appears the holds in the head have a bevelled sealing service...for a beveled plug. Not the type with the sealing washer. (term??)

Anyway...anybody have any ideas?