Howdy Mark:
There is a "Sticky" at the top of this forum that goes into this issue in detail.
The gist is that you will need to mill the '79 head enough to reduce the chambers volume to match the total volume of the '66s 52 cc chambers and .025" compressed thickness steel shim head gasket.
If the '79 head has the typical, original 62 cc chambers you will need to mill aproximately .050" to reduce the volume to 52 ccs.
You will also need to compensate for the extra thickness of today's aftermarket composite head gaskets that have a compressed thickness of .045" (NAPA) to .050" (FelPro). That cut will be the thickness difference of the old to the new head gasket. Ei- new Felpro @ .050" to OEM steel shim gasket @ .025" = .025" difference. So an additional cut of .025" for the difference in gaskets.
.050" to reduce the chambers to 52 cc and .025" compensation for gasket differences, equals a total mill cut of .075" to get your engine back to aproximately 9:1.
Now, all that's assuming your engine is OEM and untouched inside. And be ready to use hardened washers on the head bolts to avoid bottoming them out. The only way to know, for sure, what you have before milling and what you get after is to measure the chambers in cubic centimeters. There's a "Sticky on that too, over in the "Hardcore" tech section.
Hope that helps.
Adios, David
