LA small block timing is suddenly way off

OK I will build and use a piston stop to prove TDC and follow the other advice above. Although I had read somewhere that rotating your engine backwards is really hard on crankshaft bearings. Apparently that is not a widely held opinion since no one here mentioned it. Yes I do have two Edelbrock 1404 4-bbl carbs and an Indy ModMan intake maniflold. It was set up this way by the previous owner.

Based on a comment above I realize that the distributor rotates once for every 2 engine revolutions so my last night's static visual estimate of timing advance was off by 2x. A 360° circle divided into 8 sections has 45° per section. The rotor was pointing well past cyl1 distributor cap tower (about 3/4 of the way to the following tower) so last night I estimated I previously had 30° advanced timing, but that estimate would actually be 15° advanced timing relative to the crankshaft, which is a more reasonable value. Anyway I will use a piston stop and do this all again. And based on comments that I need a timing light compatible with MSD ignition, maybe that is true since I believe the MSD shoots a double spark and I don't know the relative timing of those 2 spark events, I assume my old timing light will fire on the first spark event.