Because here's the thing. There are only TWO pistons for the 340. One is the 71 dna prior that has positive deck height and two eyebrows. Those were rated at 10.2:1 from the factory, but always blueprinted around 9.5. Then there was the 72 and 73 pistons which were rated at 8.5:1 and were actually lower. MOST builds get the 4 eyebrow flat tops that do not come up to zero deck. So taking that into consideration, you in all likelihood, have an 8-8.5:1 engine, UNLESS the heads have been milled a good bit, OR you have the earlier 2 eyebrow flat tops with two eyebrows. I put my money on your engine being in the 8.5 range. Once you run a compression test, we'll know.
