an 8/1 318, with a headgasket change from 020 to 039, which is typical, brings the Scr down to 7.7.
At 800ft elevation, with the stock cam at Ica of 48*, this is predicted to make 129psi CCP.
Put a rowdy cam in there with an Ica of 64*, and this falls to 112psi, and just like that your 318 feels like a slanty, until it gets up on the pipe, which may not be until 4000 rpm, and just after that the 318 hits the wall created by the heads.
So then yur looking at a powerband of from say 3800 to say 4800.
this is a bad idea right out of the gate.
Such an engine would need at least 10.0 to be fun. But that cam is way too big for the heads anyway.
Roll it back to say an ICA of 56* and a Compression ratio of say 9.5, and now you got a low-rpm torque machine.
Anything more is gonna need lotsa dollars to make those heads work, or just get some better heads.
To get to 9.5, the chambers gotta loose 20cc, so first up, the factory slugs gotta go. Second up, open chambers gotta go.
And Thirdly, it's not likely yur gonna be able to run the 039 gaskets.
To get to 9.5Scr, the total chamber volume has to shrink from around 97cc in stock form, down to ~77cc. with special attention paid to the Quench.
Or you can just start with more swept volume in the first place, seeing as it seems your roller has not yet got an engine.....
In about 1975, I installed a 318 cam into a 340, and bolted the 318 top end onto that, stuffed it into a 65 Valiant wagon and had a killer street machine. I was 22 years old.
340s are getting harder to find, but I can see a 5.9 in your Duster, in a heartbeat.
Somebody had to say it.
You can install a whiplash into that, install a dual exhaust, and just drive it. The 5.9M is big enough that even if the pressure drops to 140psi, that is not gonna hurt real bad.