k from what i have read the 240 head will give me a gain in power but what happens when the 300 head goes on the 240? will it?
i have a motor of each of them a 66 240 merc and an 83 f-100

 

Well, it would follow that if a 240 head had smaller combustion chamber that raised the compression ration when installed on a 300, then a 300 head, with a larger combustion chamber installed on a 240 would result in lower compression. Some reason to do this? The increased CR for a 300, in the absence of any other work, would probably not be noticable.

 

There is roughly a .5 gain in cr using a 240 head on a 300. And what 85e150six4mtod said.