GM-GMC: 79 chevy k20 fuelmileage...help!, fisher plow, fuel mileage

Question
QUESTION: I have  79 chevy 3/4 ton with a 350 and only 31,000 original miles on it, and I am having some fuel mileage issues and power issues.  Right now I am getting 7.5 miles to the gallon.  I'm thinking that I should be at at least 14.  The truck does weigh in at over 5000 lbs, so i do understand some lower mileage but 7.5 is way off.  I do smell fuel when its running but can't seem to find any leaks.  if it sits for a day there will be no gas in the carb, and i'll have to use some ethanol or a dribble of gas down the choke to get her going.  also she's got no balls on hills at high speeds.  I drive the highway everyday to and from work, and i hit a hill she'll slow down to 40 and go now higher (yet she'll cruise at 80 np on a level surface). Now she is set up for plowing with the original fisher plow that was put on it 29 years ago.  And i'm more interested in low end toque.  How can i get a little more beef out of her, with out sacrificing to much fuel mileage, and do you know of any common problem areas for leaking fuel. ( those old fuel pumps possibly?)  Lemme know.

ANSWER: It sounds to me like the float is stuck in your carb. If it is still original, it may be time for a rebuild.  In an older truck like that, it really is the only thing that can affect your fuel mileage to that degree, other than driving habits.  As for getting more power, there are tons and tons of bolt on accessories from intakes to heads to cams and so on, but you will need to choose wisely if you hope to gain fuel mileage and they are often pricey.
Hope this helps.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your help.  I'll check on that carb float.  I also did find the fuel leak.  The diaphram in the fuel pump was all dryrotted.  It was leaking right into the crank case.  So I put a new pump in as well as changed the oil. And that got me up to 10 mpg in town and about 15 on the highway.  But My follow up question,  what type of gear ratio should I put in there.  I'm also assuming the being full time 4-wheel, the front end will have to be the same ratio, correct?

Answer
Yes, if it is full time four wheel drive, the difference in gear ratios would likely cause binding in the drivetrain.  Even if you were to change both gear sets etc, I would doubt that you would gain more than 2 or 3 mpg.  10/15 mpg is respectable.  I never even thought about the fuel pump causing your rich run.  Thanks for the follow up.