I ran a stock 340 cam with Rhoades lifters in a 72 318 block with 10.5 forged pistons and 360 heads and Holley 600 vac secondary carb and got 22.5" Hg at idle and 17.75 MPG highway with 2.76 gears in a 68 Barracuda....
I had 24" Hg at idle with stock 318 heads, but it had too much compression and would detonate, I had to lower the compression a bit with 360 heads and it was fine....