Not sure if small block and big block Edelbrock heads are the same but the small block heads require 5/8" hex. The spark plug recess is only big enough for a 5/8" hex.
This^^^^^^^^^. Those heads weren’t designed for that giant assed 13/16 hex. You need the Champion RC12YC or equivalent. The heat range depends on your combination.
Ditch that 13/16 stuff and you shouldn’t have too much trouble getting the plugs in.
