picked up a 340 long block w/out specs

You don't need to pull the heads to check the compression ratio and you don't need to screw with cranking compression.

Get the engine on the stand and roll the engine on the stand until the number one spark plug hole is straight up.

Get yourself a real graduated burette, not a turkey baster or one of those hardware store bull semen things. You have to move fast.

Get the piston to TDC and fill the burrette with with water and a touch of dish soap.

Fill the cylinder until the fluid reaches the spark plug hole and read the burrette. Then you can determine your CR.

You have to move quickly. Once you know the volume at TDC you can calculate the CR. If you are concerned about the fluid getting into the pan, you can use solvent, or denatured alcohol and it will dry up.

Again, you have to move quickly because you can't deal the piston with grease. I've done it many times, when some joker would come in and claim 11:1 CR and in truth he was 9.5:1 or worse.