Found this in my notes from the US site
First, the 77/78F2/3 heads shared a common head drain back hole which early K,F and late K heads did not.
This eliminated the middle drain back holes in the F2/3 heads.
The 4 studs next to the eliminated drain back holes require cap nuts and copper sealing washers on the F2/3 head only.
The late F2/3, 77/78 cylinders CAN accommodate the knock pins(or dowels) and the rubber gaskets which allows you to run these early/late K and earlyF/F1(75/76) heads.
Here's the full article which also goes into using K barrels with an F2 head
http://forums.sohc4.net/index.php?topic=99165.msg1105716#msg1105716Cheers
Dennis