OK, in order to run, the engine needs air, fuel, compression, and spark, and all of this at the right time. The #1 piston comes to TDC twice, once on the compression stroke, and once on the exhaust stroke. The plug needs to fire at the end of the compression stroke, with both valves closed. Here's how to tell for sure:
Turn the engine by hand, clockwise from the front of the engine, with the passenger's side valve cover off. Watch the #6 cylinder valves, the third pair from the front. When the exhaust valve closes, and the intake valve starts to open, the #6 cylinder will be on top of its exhaust stroke. The "companion" cylinder, #1, will be at the top of its compression stroke. Time the distributor to fire the #1 plug.
To be sure all the valves are closing, write the firing order down like this:
1843
6572
Get the valves "rocking" on #6, like you did while timing the distributor, and adjust the #1 valves to have a very slight amount of slack at the rocker arm. Turn the engine 90 degrees. #5 will be "rocking". Adjust #8. Turn 90 deg. again. #7 rocks, adjust #4. Do this until all cylinders have been adjusted. After the engine starts and the camshaft has been broken in, if necessary, go back, and readjust the valves to 3/4 turn tighter, using ther same procedure. Most of the time, it's necessary to start up with new lifters slightly loose, to allow them to fill with oil properly. After they fill, then you go back and do a final adjustment.
Jerry