Remember the cam turms at 1/2 the crank speed, and every other turn of the crank the cam WILL be 180 degrees out.

One little side note on the SBCs, with the cam marks lined up, the No1 piston will be at TDC exhaust stroke, rather that TDC compression. In other words the distributor will be firing No6 cylinder rather than No1 (one reason a lot of guys get the distributor 180 degrees out there first ime rebuilding an engine).