Originally Posted by oldbogie
Chevy has a little surprise. When you put it together with the timing marks on the cam and crank gears facing each other, the engine is set with number 6 in the firing position. If you leave it there and drop the distributor in timed to number 1 firing, it's 180 degrees out.
There plenty of other reasons but let's start with whether that is a situation or not for your set up.
Most timing sets come with a little piece of paper in them that clues you into that little surprise.