cheap/easy mpg boosters?

What kind of mileage are you getting right now?

First adjust your valves. If you don't know how there is a ton of info on the slantsix.org board

Do the HEI swap and then get the NGK ZFR5N plugs. Cut the washers off them and gap to .040

195 thermostat

2 1/4" exhaust helps a lot but isn't cheap. I don't know if you'd need bigger with a 360 or not so it might be best to not even mess with that.

Make sure you have no vacuum leaks and are pulling more than 20hg on your vacuum gauge.

Air up your tires, get rid of any pointless weight in the car and see how you do by rolling in and out of the throttle for a month.