I put the timing light on a plug lead on a 283, and I couldn't even see the marks. The engine was running well,and where the mark was only lead me to the conclusion that the pulley/damper had slipped around on the crankshaft, what else could it be? So I removed the radiator, pulled the pulley off -and all looked good. Put it all back, hooked up the timing light- the mark is now spot-on! Turned out I originally had the wire on the wrong plug (no. 2 instead of no.1). I work with electronic integrated circuits, some of which have 8 pins. Guess where no. 1 pin is ?- right where cylinder no.2 is on a SBC!