[Captainfab]

Typically head porting focuses on mainly the intake and exhaust ports with just a light clean up in the combustion chambers. If larger valves were installed, the porter may have unshrouded the valves, which could enlarge the combustion chamber. However I don't think there is enough meat in the casting to open the chambers up from 64cc to the necessary 76cc's.

Your mechanic is probably thinking he can build this 400 with the same thoughts as most use when building a 350. It is very common to build a 350 with flat top pistons and small chamber heads such as the 'camel humps'. That same thought can not be used with a 400 and have it run on pump gas. The 400 has a larger bore and a longer stroke, which pushes the compression way up as compared to a 350.

The Dart heads are good heads. However heads with an intake port of 210cc's are a little too large for a street engine. You want to be in the 180-195cc range, depending on what rpm range you want your power to occur in. If you stay with the flat top pistons you will also want a head with 72-76cc combustion chambers. Most any domestic made aftermarket head will be much better than those 'camel humps' Of course you have not mentioned what the flow numbers are on those heads since they were ported.