"then the secondaries must open fully"
The secondaries should open to 90 degrees, but the air valve may not - this is mixture bias device, not a malfunction; depends on the motor. My 455 Buick is supposed to and didn't - I filed the stop to allow it to move to vertical.
"And if it is too loose, you'll flood the cylinders"
If it's too loose, you'll have a lean stumble caused by too much air and not enough gas. Too tight and the response will be OK but you'll lose some flow - but with a 318 you have more than you need anyway.
The usual pre-load of air-valve tension spring wind-up against lightly closed is about 1 full turn (360). If it bogs when fully warm, tighten it a very small amount and try again.