are the exhaust ports higher on the G heads? That is good, and a truck has lots of room for headers (headers help with gas mileage too), as does carb choice.
Id choose on which heads are in better shape, or on what compression ratio you need. the 66 heads may be 72 to 74cc and if the other heads are C8's they may be 68 to 70cc. depends on chamber. smaller is usually better for a 352.