[66fastback]

this can be a great swap, but not all 351W heads are created equal. Some can be of almost no benifit at all .....the ports and valve sizes on the mid 70's heads are about the same as what you already have. You are also right to assume that there can be an increase in combustion chamber size, which would cause you to lose compression.

The answer has always been to find a pair of 69 or 70 model heads. I'm sure there are other years that are as good, but these used 58 or 60 cc chambers (as gofastmercury pointed out) and had the bigger ports and valves.

The bottom line? these heads probably give you about the same increase as the GT40 iron heads.....20-30 hp. Consider well the difference, I bet GT40 irons won't cost you much more than finding good 351W heads and rebuilding them. I know everyone is talking about the GT40P's, I'm just not sure about headers available for them, as I think there is a problem with the spark plug angle and clearance.

either way you go, good luck. The heads really make a difference you can feel.