You can buy a nice brand-new Australian-made damper with two timing marks (one of which will match up with your driver's-side timing indicator) from Falcon.
Things still seem a little murky, knowledge-wise, on exactly how practicable or wise it is to use '76-up (cast-crank) dampers on '76-down (forged-crank) engines or vice versa. All slant-6 dampers physically interchange, but it seems likely there are functional differences. Looking at the '82 Dodge Truck FPC, for example, I see three different dampers listed: One for manual transmission with 10" clutch, one for manual transmission with 11" clutch, and one for automatic transmission. The belt drive at the front of the engine doesn't know or care what kind of transmission is bolted to the back of the engine, so that's not it. But the mass, inertial characteristics, and resonant frequencies of the flywheel for an 11" clutch, a flywheel for a 10" clutch, and a torque converter/flex plate can reasonably be expected to be substantially different. So the difference in vibration dampers is likely different vibration-damping performance profiles. And that's just for a cast-crank engine. There's additional discussion on the matter here. Keep in mind that the vibration damper's job is to prevent the crankshaft breaking, not to address vibrations you can hear or feel from the driver's seat. It's possible that some crank/damper combinations other than those intended by the factory may work without any problem. It's also possible some of them may seem to work fine but fail to damp (or may even amplify) crankshaft torsional vibrations at a particular frequency. If you happen to hit that frequency by hitting the "right" engine speed and load, the crank may break.
We need more solid, reliable data, but I have a hard time imagining how to get it.
Note 1: In Australia where the linked damper is from, all slant-6 engines had forged cranks.
Note 2: The linked auction for the 3-groove damper has incorrect years listed; that damper does not seem to show up in '74 or '75 FPCs, which calls into question the seller's statement that it is factory equipment on both forged-crank ('74-'75-early '76) and cast-crank (late-76+) applications.
