850 Mighty Demon on a 388 motor????
What were you thinking?
Who recommended that?
Annular boosters?? Is this a 4000# Truck?
I'll bet you had a tuning nightmare!
I run a 375" motor with a Race Demon RS set up at about 720 CFM and when Bo Laws Flowed it, it made 817 HP on a 540" motor but of course dropped off at 6700 RPM as it just wasn't a big enough lung for the big inch motor.
850 need a 450" motor or bigger.
Bo installed a pair of their super trick metering block and it picked up 14 HP over the out of the box, properly tuned set-up.
$550.00 is dirt cheap for what you get, you can call Brazwell or BLP and maybe get a better carb but be prepared to spend double or more for 10-20 HP.
Read this book
www.4secondsflat.com/Demon_Tuning_Guide.html
You'll see why a 850 is too big for a 388 motor and learn how to tune it when the carb is right for the application.