To my knowledge its only needed
on blocks older then 1981. Me and my engine builder went round and round over this issue and I let him talk me out of it.......big mistake on my part.
Chances are if you needed to drill yours you`d already know. Mine heats up in like 10 seconds or less when I flog it, and every part is new, including radiator.
Here is part of an email I received from Edelbrock tech on the issue.
The cylinder head has a large, roughly kidney shaped, water passage cast into it that feeds the steam hole, it is not a separate distinct hole. Our heads are reversible, there is a hole in the appropriate area on both sides. Please note that the driver’s side and passenger’s side holes are in different positions. The steam hole would be drilled in the block, it would not have to match the shape of the hole on the cylinder head. Although I am uncertain of the precise consequences of not drilling that hole, I would have to assume that it would cause a significant cooling issue.