Running the same carb on my 68 340 cuda, had to jet down to 66 with a 4.5 powervalve in the primary & 72"s in the back with no pv. Idle screws are about 1 1/4 turns on all 4 corners. Running mopar purple cam, 260/260 @ .50. Hope this helps.
I bought a unfinished 68 barracuda, installed the 8 3/4 rear end which came with the purchase, I switched the 3.91 chunk out for a 3.23 before I installed it, no bottom end with 3.23 gears, a 340 does not have that much torque to begin with, big mistake.