Is it possible the engine has hydraulic lifters? If there's been an engine swap from a Powerglide-equipped car sometime in the past, you could have a hydraulic-lifter engine. If so, there will be no gap at the rocker arm, even at TDC on the compression stroke. The rattle you're hearing on startup could be collapsed hydraulics pumping up.

Try this- - - -warm up the engine, then pull the valve cover and line up the timing mark and distributor rotor the same way you did before, them use a long screwdriver or a pry bar and put a firm pressure against the pushrod side of one of the #1 rocker arms for a couple of minutes. If you've got hydraulic lifters the gap at the valve stem will increase to about 1/8" or so. As soon as the engine starts the lifter will refill.
Jerry

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