Jode, that vac. advance is #1 to fix. Then check the mixture and tune it a little bit leaner than what you have (if you smell a lot of gas when running). What gears are you running?
If you have 2.72's through 3.31's, with your tires and 4bbl, you should be getting around 10-12mpg. average if you do some highway or straight driving and some traffic driving, with a regular foot (in it about 15% of the time).