The timing chain is assessed as follows:
Remove the valve cover. Between the chain's cam sprocket and the most forward cam support is a collar which has a notch in its outer radius. On the left side of that front cam support is a groove cut C/L to the cam. Turn the engine NOT by the cam but by the power steering pulley's nut so the notch on the collar is aligned with the groove in the tower. That's the cam's TDC. Then read the crankshaft's angle off the harmonic balancer. Replace the timing chain if that angle exceeds 5 degrees; i.e. that the crankshaft is 5 or more degrees ahead of the cam.
Know that the chain also drives the injection pump so it's best to link the new chain to the old one and wind the replacement chain through, keeping tension on both ends so the chain's links don't jump a tooth anywhere. If it jumps the IP and the cam must be put back into sync with the crank.
Suggest you look through the archives at
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and check the diesel forum where there are numerous owners of sixes like the one you're about to repair.