Warren.  I don’t see an answer to your question and after looking up the amount to mill in the 1955 Service Manual, I don’t find a definitive answer there either.  In the ’57 manual, head milling is limited to 0.010” and most of us will go with as much as 0.025” of mill on the ’57 unposted ‘G’ heads.  The posted ‘G’ heads I’ve taken as much as 0.065” and I’ve seen as much as 0.080” on the posted 113 heads without any head gasket sealing issues.
 
With all that being said, I would treat the 1954 heads like any ’55 head in that I’ll hold head milling to a 0.050” max number.  The ’55 heads are designed somewhat differently than the highly touted ‘unposted’ G heads so the ’54 heads can likely be treated the same as the ’55 heads.  The 1955 272 "182HP Special" heads were factory milled 0.030" so that should represent a very safe number.  I would suggest checking the combustion chamber volumes on both of your heads and then mill both heads accordingly so that they are both equal in cc’s.  Give a shout if you need some help with the number crunching on that.

Lorena, Texas (South of Waco)

Hey guys... I'm rebuilding a 239Y right now, I have the heads off, everything is marked EBU.
How much can be milled off these heads ..?..   Or, what do you guys recommend ??
I drive my '54 a lot and far sometimes, I love the great gas mileage of the 239 and want to stick with it.
I'll follow your expert advice, no arguments !!    :-))
Thanks much,
82warren