Well I am still looking to buy a truck while I was looking I came across an 87 GMC Serria Classic. Its a little older than I was looking for but it suits my needs and my price range. Most major parts are rebuilt, rebuilt 350 with a 4 bolt main, tranny, new drive shafts and so on. There are just a few problems. It is higher mileage(around 180,000) the driver side door is dented up pretty bad and the compressor on the AC is out. I have a couple questions for the arfcom gearheads. First about the engine a rebuilt 350 with a 4 bolt main, what is the estimated gas mileage and about what range would the horsepower be? We have figured around 12mpg and I dont want to spend to much on gas. What would it cost me to fix the compressor for the AC? And the final one, I doubt you can know for sure but could I expect to pay for a new door? Thanks!

Bump- Nobody knows about gas mileage or a compressor?

15 mpg, and a compressor will cost you about $250 for a rebuilt.  350s are tough and lots of pwer, just no winner on gas mileage.  Is it 4x4?  Ops

15 mpg is better than I expected to hear. It is a 4x4, it has a 4 inch lift and 33in bfg mud terrains.

Look VERY carefully at the cab floor mounts under the cab floors for rust.  The 33" tires may cut your mileage a bit.  Does it still have the TBI injection system or has someone replaced it with a carb?  Ops

I belive it is TBI. But I am not sure on this.

15 sounds way too optimistic.  I have had dozens of Chevrolet trucks and have never had a 350 make better than 14 on a long trip.   More commonly I see around twelve with my newer vortec. And about ten with the older throotle body and carb rigs.  
Aftermarket sheet metal for that body style is dirt cheap.   I would expect about $250 for an AC pump.  But ,  the R12 needed for that system is probably going to run you that much as well.  Or you could convert it to R134.  But this would probably require you to change out more than just the pump.

12 to 17 mpg, they vary, i have an 84 silverado with over 230k on the truck, on its second motor with about 150k on it everything is original except the motor and trans still has all the factory front end parts and uv joints best car/truck i ever owned by far, parts are cheap and a dime a dozen for small blocks. The only complaint i have is the wiper motors/washer pump is a piece of shit and the heater fan switch will eventually short out.