fattest cam in a 340?
You've already covered the one that messed with me the most back in the day, a cracked distributor cap.
could have a pre '71 damper on it. see which side the mark is at TDC.
Actually, its more of a mismatch between the balancer and the front cover. 64-68 LA engines used a tab as a timing mark. 69-70 had the marks cast into the front cover. In 1971, the water pump was redesigned requiring the timing marks to be moved from the passenger side to the driver's side of the engine. If a pre-71 front cover and a post-71 balancer or a pre-71 balancer and a post-71 front cover are used in combination, the timing mark will be 90º off.
Another possibility is that the timing chain has slipped. The OEM, and several aftermarket, timing chain sets use plastic teeth in the camshaft gear. Like all plastic, it gets brittle and after a while a tooth will chip out. Since you're going to pull the cam anyway, think about using an all metal double row timing set when you go to button it back up.
How to find TDC without taking the engine apart, assuming the cam is correctly timed. It's not exact, but it can do until the timing set can be accessed. Remove the valve cover over the #1 cylinder. Remove spark plugs. (Actually only need to remove #1, but pulling more makes it easier to rotate the crank.) Cut a straight piece of coat hanger wire about 1½' long. Put wire through spark plug hole until contact with the piston is made. You may need to bend the wire so it will rise and fall freely within the cylinder. Rotate the crank until it pushes the wire to its highest point and both valves for #1 cylinder are closed. If the wire is up but both valves aren't closed, another full rotation is needed. Both valves are closed when the rocker arms are loose against the valve stem.
