Just curious, why would the exhaust bolt pattern be different. You mean you cant use standard CMIs, IMCO, etc? If you mean they moved the whole pattern then they also moved the port.
I would be all for that because a few years back I spent three weeks discussing this with Edelbrock engineers convincing them that some marine exhuast manifolds (i.e. Stainless Marine) would not properly fit their heads due to interference issues. They ended up paying me for custom adapters. Great guys to work with BTW. Moving the port would change the flow characteristics of the head however.

Also, Edelbrock cast the the heads for GM Perf and have the GM logo on them. I'm still thinking they are basically the same overall casting.

Although I have not taken any measurements, I have heard that a standard set of BBC exhaust manifolds/headers will not directly bolt on these heads.

There was a set for sale on e-bay (it might have been somewhere else)a few days ago w/the mercury logo cast in them,i'll do a search and see if they are still kicking around, from the pics we could see if they are ovals or rectangle port,Smitty
Here is the pic off e-bay,i found it under "mercury racing" BUT they don't say in add exactlt what engine these go to but they are rect port,hard anodized,they have mercury racing cast in them AND the bid on them is only 850 w/just over 24 hours left on auction if someones interested!

The heads are custom made for Merc. The headers do have a different bolt pattern than the other BBC.