My 318 used to knock for a minute or two after it sat for a long time. Since then I have just disconnected the coil and, watching my oil pressure gauge, crank the engine until the oil pressure builds up. priming the engine if you will. Connect the coil wire and fire away. It is also easy enough to pull the valve covers and check for a loose push rod. It is a piece of mind. Do you have adjustable rockers on that 340? If not, and the pushrods are not loose, you may have a lazy lifter that is taking its time pumping up. You would want to replace that. You would know if it was something more serious. A knocking connecting rod is a horrid sound. Good luck in your diagnose. You must preserve that nice 340. Wish i was close. I would come over and help you.