I asked this question of Cometic through email.
They said "The part meant for the Edelbrock heads has a slightly different water jacket embossment that fits that particular head better than a standard part. I would use the C5917."
I just got mine running so I have no long term comments, but it did match up pretty well to my Edelbrock head. Mine is the Magnum version Edelbrock head. I need to find which part number I ended up using.
Page 36, post 894 on the following thread has a picutre of a C5622 from another discussion on this topic, so you can comparte to your heads:
ALERT!! SOUND THE ALARM!!..
Post 884 in that thread had a great suggestion to just call 4secondsflat, give them your application, and he will help get the correct one, which may end up as a custom part number.