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

[special-K]

They had 350s w/454 an option.
Turbo400=electric kickdown/Turbo350=cable kickdown.
Could it have shorter tires causing the speedo to show a higher speed than actuall?Or...it could have the wrong speedo derive for that transmission.It woud have come with a TH400.So,if it`s got a 350 it could be from a 1/2t with 3.42s or even a 10-bolt 2.73,or whatever that super tall ratio they used.
I personally don`t feel any love for Edelbrock carbs.Is your`s a Q-jet replacement,as in spread bore?The Q-jet with smaller primaries,when set up properly,will yield far better econmy that a square bore.At least that`s been my experience.For years Q-jets have been toss for Holleys and now Eldys.It seems many are out search for the Q-jets to go back on,since GM got it right as they seem to do so well.

[pig rig]

they have even been know to have strait 6 in them
most of the duallys came with 4.11 gears in the rear

[Devoncs1]

Well idk its not the orig. transmission but it's a turbo 400. (I looked at the oil pan)
I'm sure the speedometer is off, it's off by 7 mph, for some unknown reason, but that is not the cause of the crazy high rpms. As I said, going 70 on the interstate it's around 6k rpm, and just cruising at 50 it's around 4.5k.

[Damien]

The vacuum leak will cause the high idle. So yes, If you replace the carb, it should run better assuming that is the only problem.