Alright, another Alfa in the LBC!
Welcome to AlfaBB and to Alfa ownership BM! 1974 Spiders are among the most sought after.
This headlight switch dilemma sounds familiar from my Spider days.
1) I seem to recall a total of 3 positions on that stalk switch, off, parking lights and finally headlights.
2) I believe the headlights will come on when the stick is pushed in while in the off position and parking lights position. So you can flash at those slow suckers in the fast lane and get them the hell outta the way.
3) Pulling the stick down while the headlights are on should turn on the high beams. You probably have a partially blown bulb. You can pick up replacement 7" sealed beam headlights at any parts store. I picked up a set of H4 Hella bulbs which are much brighter, yet not blinding to oncoming traffic.
Post some pictures of her in the gallery!