It also depends on the heads. They changed the design slightly in some. The 6X-8's I have on my 455 right now this chamber is not open to actual the exhaust ports, so I didn't have to block it off and with the Edelbrock intake the top 1/4" of the opening is showing and open with out any issues... so you may or may not have issues if it's not blocked off properly depending on your configuration. Best bet is as Merv suggests and get the Felpro gasket set.

Brett!