Bad mileage, high rpm, high idle?

[Devoncs1]

Hi, I'm new here lol.
I have a couple questions here, hehe.
First, my truck is a '79 Chevy C30 silverado 1-ton dualie (i've only had it 2 weeks maybe).
It has a 350 in it, with either 65k on it, or less (the ode. says 65, so the engine could have that or less, i have no clue).
It's not the orig. motor, i believe it came with a 454, i don't even know if the dualies came with 350s, lol. Anyway, the trans is not the orig. one either...
Its an auto so i figured a turbo 3450, but I'm lost as to how fo find out, anyway...
The edelbrock carb on it seems to be a piece of crap... I'm going to put a quadrajet on there(the edelbrock has a vacuum leak straight through the center on both sides...). I know this truck is geard low, but my experinces hace been that going about 70 down the interstate the engine is revving about 6k, likewise it sits at 4500 around 50 mph, lol. Is this right at all? it seems grossly high even though it's only a 3 speed, also the thing idles at about 2500, ive tried toying with the air/fuel mix on the carb to no avail. It changes it slightly but not anything crazy.
Also the truck has drank about 30 gallons of gas in around 200 miles, which is nasty...
So what i'm asking is.. are the idle, fuel economy, and rpms going to change with a new carb, or are all these things par for the course?
thanks in advance