i'm looking to put a cam lift in my 383 roadrunner. I'm running headers and exhaust besides that the motor is pretty stock. whats the best lift duration for this setup?

 

what is your rear end?

 

8.75

 

what ratio?

 

i cant be certain but i'm pretty sure it's factory

 

ok
jack the back of the car up
put the car in neutral
mark the bottom of the rear tire
and mark a spot on the driveshaft
use chalk
make one revolution with the tire, and see how many times the drive shaft makes a complete revolution
that will give you your ratio

by the way, do you have sotck rims and tires?
if not, you wont get a reading that was original to the car, but it is ok