Mossy, sounds like you have a voltage drop somewhere.
Check over your harness and clean the connectors. The light should work fine, the electrical load is only a little higher than stock.
I have the same bulb in my '75 750F, no problem, however I do use a relay in place of the starter button which failed before I got the bike. The problem could be there, I think your headlight is run through the starter button also.