When I close the doors on my '71 Chevelle, 2 door HT, I can still move them in and out about 3/16" to a 1/4" inch. They're latching, but not tight. The hinges are OK, and not loose. Do I adjust the door by the locking mech. on the door, or by the pin on the door jam? And how do I do that?

On my 64, there are rubber blocks in the jamb area that act as stops, also the rubber door gaskets keep pressure against the latch pin.
The latch on the door does not fit tightly to the pin on the jamb without something (eg: gaskets) to take up the slop.
With no gaskets, the doors are really loose.
My new door gaskets took over a week to compress enough to let the door latch all the way - just an idea of the pressure they can create.

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Make it the way you like it, forget what the other guys say!
http://chevelles.com/showroom/alelcaminofront.jpg
http://www.angelfire.com/al4/alcamino64/index.html

DDinNY the pin in the door jamb is movable. loosen it, move it towards the center of the car and tighten it again. There is a finite amount of adjustment there but you never know. It might just tighten up the doors a bit.