Does the engine run better with electronic dist?
If timing is more "jumpy" after dist repl then it might be your dist. Pop the old unit back in & recheck. Look for wear in dist shaft and drive gear teeth(it's plastic after all). Also check for loose dist pickup coil or vac advance.
With dist in eng, gently twist the ign rotor with your fingers. It will be "solid" one way & "springy" the other - as it loads the mechanical advance. There should be almost no loose twist between the solid & springy points.
To check your timing chain, you need a good (not sloppy) dist in the eng & some way to slowly move the crank pulley by hand. If you can use the fan blades & alt belt - then great. Most /6 didn't have a center crank pulley bolt so you might have to loosely screw a couple bolts (3/8 or 5/16 course thread) into the smaller holes of the pulley & use a pry bar to spin the eng over.
Now turn eng clockwise to bring the crank pulley to TDC & mark the dist housing where the rotor tip is pointing. SLOWLY move the crank counter clockwise until the rotor just starts to move (you are taking up the T'chain slack). Measure how far the TDC mark on the crank pulley has moved.
1/8" or less - dude, you're golden ; 1/8-1/4" - eh, so-so ; over 1/4" - it's time to consider doing a chain.
Some tips:
you really want (need) a puller AND installer to remove & refit the crank pulley. The hole in the crank ( for the bolt that likely ain't there) is a 3/4 fine thread. Trying to beat the pulley on with a hammer don't work too good (ask me how I know).
You can probably finagle the timing cover off without pulling the oil pan, but you probably can't get it back on (without big oil leaks afterwards) unless you drop the oil pan - & that ain't no fun.
Timing chain condition depends on what the engine was originally. Most OE truck /6 had iron or steel chain sprockets & were pretty durable. I don't remember if passenger car eng had the crappy plastic & aluminum cam sprocket that so many car makers used in the 60's, 70' (&80's!)
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