Here's the drill: a 30 over 318 comes in at 323 cubic inches. Doing the math each cylinder has a volume of 662cc. To get to 9.5:1 compression ratio you need 77.8cc above the piston.

Assume that by the time you get your heads ready to go they have 63cc volume.

Use a Mr. Gasket thin head gasket I believe it's about 5cc. So you need about 10cc more, that puts the piston 0.050" down in the hole.

Say your block specs in at the standard 9.600, you have 6.123 rods and 3.31 stroke, the piston pin will be 1.822 down in the hole. So with a flat top piston you need a compression height of 1.822 - 0.050 = 1.772 to get to your dream.
There isn't a single 318 shelf stock piston with anywhere close to that compression height. The tallest 318 LA piston is for the '89 - 92 Mag motor, comp height 1.755. Slightly earlier is 1.745. Both are flattops.

The first piston with valve reliefs is 1.720 comp height. Plus the valve reliefs add maybe 4cc to volume, lowering the compression ratio even more.

There's even a piston with 1.658 comp height for really low compression engines.

The three most dangerous words in engine building, or life, are "I have heard".
Coated pistons are bad? Hypereutectic pistons are bad? Then why are nearly all of the new engines being built today with pistons made of a hypereutectic alloy and coated? KB is bad? They have a thriving business and have had one for years, surely if they were so bad they've have starved to death long ago. Speed Pro is bad? Their pistons are heavy it's true, but they are virtually indestructible. And they are built by Sealed Power, the company who has for the last half a century or more supplied a large percentage of OEM pistons to American manufacturers. Speed Pro forged pistons are the old TRW forged pistons. TRW sold decades ago.

The only shelf stock piston that comes anywhere near doing what you want is the Keith Black, or you can always go forged. You can get a custom forging any way you want, as long as you can pay for it.

R.