86 GMC PU with new crate 350 getting 8 MPG.

[dyno]

So I got my boy his first truck, 86 GMC 1/2 ton 4x4. Just put a new crate 350 with Performer intake, Headers, Edelbrock carb. No matter what we do, we can't get any better than 8.2 MPG. It has a 4 speed with 3:73 gears and running 35" tires. Hell, my old Suburban with a worn out 350 running 35's got 13 mpg!

Do these crate motors get better MPG after they are broken in and loosened up? We only have about 500 miles on he motor.
BTW, it has the stock cam.