very weak spark

Relay has nothing to do with ignition voltage

I would not immediately blame the "first guy." "New parts" do not mean "functional."

So far as 5/7, it might be that wear in the distributor is "just right," or even a bent shaft, that those two don't open the points, or the opposite, the points never close on those two.. Wiggle the shaft, recheck point gap. What did you set dwell at?