As John points out, the ‘G’ heads were available on the 272’s and the only issue with running them on any Y engine would be when running the valve lift much higher than stock.  By design, the intake valves on the Y-Block heads are positioned such that they will hit the cylinder walls thus preventing them from free dropping into the cylinders.  While this has likely saved many an engine from catastrophic failure, this does limit the amount of amount of valve lift you can safely accommodate without taking appropriate corrective action.
 
For Y builds using oem iron heads, I use the 0.535” intake valve lift number as the maximum lift that can be used without having intake valve to cylinder wall clearance issues.  Any intake valve lift values higher than that requires that the cylinders to be notched for intake valve clearance.  Here’s a picture of what happens when you put too much lift where the cylinder has not been notched to accommodate it.  That’s easily corrected by simply notching the cylinder walls appropriately.  Just don’t notch too far down where the top ring rides in the cylinder bore.
 
  
And here’s a picture of the notch that alleviates the problem.
 

Lorena, Texas (South of Waco)

Thanks!

G headed valves will clear the 272 bore, Ford put G heads on 272s in '57.

John - "The Hoosier Hurricane"

Yup, I didn’t know anything about sonic testing when we did that.

  Dennis in Lititz PA

272 piston are.125 over 239.

Who wouldn't want to stay in "Le Tits"?  I know I would. 

John, I have G heads on my 239 since 2013. Standard bore 272 flat top pistons and a 312 crank.

  Dennis in Lititz PA

Won't the intake valves crash into a standard bore 239?  Maybe the bores would have to be notched to clear the valves.  Dennis, I realize your bores are no longer standard, have you ever put G heads on a standard bore block?

John - "The Hoosier Hurricane"

Thanks for the reply!  A funny story on myself: Years ago, I stayed overnight at a motel in Letitz. When checking out in the morning, much to the amusement of the two female desk clerks, I asked where there was a good place to have breakfast in your town; however, I pronounced the name  as "le Tits". They giggled and tried several times, to my discomfiture, to teach me how to say it, without success. Finally they helped me out by making me repeat the word "lettuce". Then I knew how to handle the name while I was in town and not embarrass myself further. I guess that what happens to southern boys when they wander up north! 

What version 239 do you have? ‘54 car or ‘55 truck. If you have an early 239 the water passages won’t line up. You need to drill a water passage in the block at #4 and#8 also weld the little triangles to support the new head gasket..

  Dennis in Lititz PA