therein lies the importance of marking and at least ensuring on removal the position of the rotor when on TDC..and also NOT to rotate said crank till you have reinstalled the distributor. Wish I had a nickel for every one that was "absolutely not 180 out" Factory timing which is the relative 7 position is obtained by proper installation of the oil pump to the cam. While the engine can be timed at any point on wrong insertion of the oil pump, one must make this mental note and clock the towers as needed to compensate. However I have never thought it practical to deviate from proper assembly as such time down the road it often will rear its ugly head in such cases as this....you now have the opportunity to place things back proper and thus be able to with some degree of certainty, follow the book. TDC is established and verified by the access hole in #6 cylinder postion on the head and by valve to determine if TDC 1 or 6 at that time. You can short cut by doing TDC and if it does not start immediately, rotate the shaft 180 and not bother to check the valves with access plate removed or doing the compression with finger as you position the crank TDC. This #6 access is ball park only as only a pison stop and degree wheel will establish true TDC