Dan, I’d reply pretty much what 318WR said.
“A little more zip.” Is a very broad and general term.
This is what I would do. First! Find out if the Duster owner is willing to use headers.
Second, use the step by step KISS program. This starts with the basic but of on parts and a cam last. The following is my suggestion to this process.
1st, are headers going to be used?
Ether way, headers or not, install a dual exhaust n the car with the muffler or choice. In a blind and broad decision here, use 2-1/2 exhaust pipe size. If you use headers, go with the sun I exhaust package. Relatively inexpensive and complete.
Works great!
2: A 4bbl. carb and intake. Again, in a blind choice, a RPM or Weiand Stealth with a 750 cfm carb of choice will be an excellent set up.
3: Upgrade the ignition system.
4: Camshaft swap. This has to work with the rear gears and the converter.
After this path is completed, the owner can decide if the next stall is needed. It comes as a toss up between an overdrive transmission or rear gears.
On cylinder heads, the stock ones, not matter which ones, can be rebuilt and given a competition valve job. That’s all that’s needed for a low lift cam under .500.