Have you read the color of the plugs you are running now?
Can you go up 1 heat range?
I've seen a good guide to reading plug mixture and heat range, and can try to find it again if you'd like to see it.
EDIT: This is a link to a good article about reading plugs for mixture and heat range - http://www.angelfire.com/fl4/pontiac...Readplugs.html It gives you some idea of what to look for, but to really get an accurate reading you need to have fresh plugs and do a "plug chop" where you run the engine at its operating RPM (usually WOT for racing) and then shut it off immediately - not really the same type of situation you have. It still may give you some indication if you can make a plug change to increase efficiency. I would expect that you could go 1 range hotter because of your consistent low RPM running.
Jay