Just a minor note:
Since the days of leaded fuel have long past, I had gotten largely oblivious to plug maintenance (my Jap grocery-getter goes 100K).  Since the Bird was running well, it wasn’t until a recent seeming unrelated (leaky steel head gaskets/coolant pressure) issue incited a plug change.  For quite a while now, the car had been rather ‘bogging’ off the line, and I was thinking it probably was time to again change accelerator pumps.  But, after recently installing a new set of indexed plugs I had put aside years ago, now when I tromp the throttle in low gear, the tires chirp! Go figure.

6 VOLTS/POS. GRD. NW INDIANA

I wouldn't think that indexing the plugs would make that much difference, but new plugs could. 

Obviously. Unless the pump-seal had been loose, and suddenly re-seated itself.
Side benefit of the indexing was the numbered boxes (easier to keep track of which plugs came from what cylinders). 

6 VOLTS/POS. GRD. NW INDIANA