measuring timing chain stretch

This weekend coming up, I would like to measure the timing chain stretch on my 6.9 ('79)
I've been told you simply pull out all the plugs, rotate the engine till its
at tdc on the crank pulley then you pull off the right hand cam cover and check how many degrees the cam shaft is retarded... sounds easy enough.
what am I going to see, a line on the camshaft sprocket and a line near it that it should line up with ??
the left hand cam cover gasket is leaking, so I'm replacing both and I wanted to do this while they are off.
And to change the chain, you just have to undo the tensioner on the left side of the engine, link the new one to the old one and feed it though, almost sounds too easy, is there a catch ?
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The marks are behind the sprocket on the bearing tower

It's a 2-man job. You will want to turn the engine by hand, and hold the chain tight.
You will also want to protect your hand as the chain will tear it up.
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Thanks

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On the old 617 five cylinder diesel the chain replacement threshold is 5 degrees.
How many degrees is too many for this V-8??
Replacing the chain isn't fun so it would be good to know if replacement is really needed.
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any more than 4 and its getting replaced, thats my threshold :)

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