If you want the engine to last, moly rings are a no-brainer. You do have to have the honing done with moly ring spec roughness.

Compression height is the distance from the centerline of the wristpin to the top of the piston, not counting any dome or dish. It is useful in calculating things like distance below or above the deck.

Your '71 318 has a deck height of 9.600". That means from the centerline of the crank to the top of the block is 9.6". Later LA and Mag engines used a slightly shorter deck, 9.577, to make up for the change in head gaskets used. Let's do some math: The height of the stroke/rod/piston usually equals or is less than the block deck height. Stroke is 3.31, half that is 1.655. Rod length is 6.123. Piston #1 has comp height of 1.741. 1.741 + 6.123 + 1.655 = 9.519" Piston is 0.081 below deck.
Piston #2 has comp height of 1.810.
1.810 + 6.123 + 1.655 = 9.588". Piston is 0.012" down in hole.
Piston #3 has comp height of 1.755".
1.755 + 6.123 + 1.655 = 9.533". Piston is 0.067" down in the hole.

There is really no reason to use the 1.741" comp height piston. We sweep that off the table. There are also pistons with 1.72" comp height, those don't matter either.

That brings us to the KB167 and Sealed Power H814CPs.

The pistons I'd like to use are the KB167s. They are lighter and have valve reliefs (have to, because they run so much closer to the deck.) If you were going to run aluminum heads, they'd be a no-brainer. But you might be considering using your stock heads, which have about a 71cc open chamber. Using a 0.040" head gasket you have 12 cc plus the head, which adds up to 83cc, plus 5cc for the valve pockets makes it 88cc. A 30 over 318 cylinder has a swept volume of 661.6 cc.
(661.6 + 88)/88 = 8.52:1 Not too bad for an open chamber iron head.

But there's another way to roll and it's the H814cp. With this piston you use the 302 heads from a late '80s Fifth Avenue, and a Mr. Gasket 1121G thin head gasket. The 302 heads are 62cc, the gasket is 6.2cc, the volume above piston but below deck is 13.4cc. So total volume above piston is 62 + 6.2 + 13.4 = 81.6cc.
(661.6 + 81.6) / 81.6 = 9.1:1 compression. That's very doable with the 302 heads' heart shaped chambers. That's what I'd pick. Why? The H814CPs are a stock replacement piston and can be used without rebalancing. They are available on Ebay for $132.00. The KBs run a minimum of $320.00 from Campbell or Hughes.
The KB pistons weigh about 90 grams less than the 814s, and though that's not bad, it means that besides the $190 more you pay for the KBs, you really should rebalance the engine at another $200. Then too, either set of heads will need to be gone through, and the 302 heads already have a hardened exhaust seat area. This has to be added to the '71 heads, meaning more cost to use them.

Because the 814s are down in the cylinder a ways, the valves won't run into them with the cam you're thinking about.


My story: My last engine was an '86 318. I kept the stock pistons, which have a comp height of 1.745, so on my late block they were down 0.055". I used the thin head gaskets and the stock 302 heads and it'd run all day on regular unleaded. My heads didn't need much work, I checked all the valve seats and guides, added new valve stem seals and new springs, and then I went wild with the grinder in the ports but you don't have to do that.
Using a Holley Street Avenger and a cast iron 360 intake I was surprised when the first tank of fuel mileage increased from Poly 318s 15 to 20!

So that's the story. Good luck on your build.

R.