Timing Retarded

Really the only way that is PRACTICAL and accurate to check timing marks is a piston stop. There is all kinds of nonsense with balloons, liquid gauges, etc.

So far as the firing stroke "if it had been running" it HAS to be on the firing stroke.

What got you to this point, IE not running?

I agree, though, sounds like the chain may be worn or slipped. I believe there IS a procedure in some of the shop manuals for verifying cam timing. You have a manual?

You can download a 66 manual here

http://www.mymopar.com/index.php?pid=31

direct link

http://www.mymopar.com/downloads/servicemanuals/1966_Plymouth_Service_Manual.zip

but the page no's do not follow the conventional "dash" numbering

69 and 72 manuals. Engine/ drivetrain pretty much the same:

69 Dodge Serv. Manual

[ame]http://www.abodyjoe.com/pictures/Misc.%20car%20info/69%20dodge%20service%20manual.pdf[/ame]

70 Dart Challenger manual

[ame]http://www.abodyjoe.com/pictures/Misc.%20car%20info/70%20Dart%20Challenger%20Serv%20Man1.pdf[/ame]

72 Service Manual

[ame]http://www.abodyjoe.com/pictures/Misc.%20car%20info/1972%20Plymouth%20Chassis%20Serv%20Man.pdf[/ame]