I've used both and never had a failure of either type.Flow doesn't suffer much under .500 lift if using the 318 sized. These days I just take 15 minutes and port match into the head port 3/4 of an inch to the 340 size. I fill the port with modeling clay and use a shop vac when port matching heads on the block.
I'm looking for a head flow chart I did comparing the two different size gaskets on a set of 302 castings and will post it as soon as I can find it.
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