Howdy Daniel66:
If you haven't already read the sticky post at the top of this forum please read it. If you have, you might want to reread it.
In summary, the '83 head will have several advantages over your broken '66 head. The down sidesof swapping to a late model head are, a larger volume chamber on the '83 head and a thicker aftermarket head gasket gasket.
Measure all critical dimensions for yourself. In all cases it is best to measure both before and after machining. The following is a good generality, but with generalities, there are always exceptions. With FoMoCo, "Always" and "never" are dangerous words.
The stock '66 head chamber volume is aprox 52 ccs.
The stock '83 head chamber volume is aprox 62 ccs.
.010" cut of the head = slightly less than 2ccs.
Late heads can be safely milled a total of .090".
Stock OEM steel shim type headgasket measures aprox .025".
Aftermarket composite type head gaskets measure aprox .045" to .055" compressed thickness.
My advice- Use the '83 head, and have a valve job done that will include new valve stem seals, a three angle performance valve job. Mill it .025" to compensate for the difference in head gaskets, and .050" to compensate for the difference in chamber size. Use hardened washer on the head bolts on re-assembly. This should equate to about a stock 9:1 CR or slightly less.
Now you're ready for an electronic ignition, a bigger carb and an improved exhaust system.
Enjoy the ride.
Adios, David
