The Hemi AFB had angled mixture screws, not horizontal ones that later AVS had. Hidden screws only on the front starting in 68 with CAP pollution control.

Maybe semantics, but the screws are the idle mixture screws for getting air fuel ratio at idle and off idle correct for minimum emissions. And while yes the adjustment will effect idle, it is only to a point. Your really shooting for the correct air fuel ratio. Once you have both carbs dialed for idle mixture, you want to set the "idle screw" on the front carb so it is contributing 50 or so RPM to your idle set point. This is the way you get the primary blades of the front carb open some to contribute fuel and air to help prevent starving the front cylinders. Having the "idle mixture screws" correct but blades closed really won't contribute to helping get fuel to the front cylinders.

The 66-67 carbs did not have hidden screws, so both could be adjusted. But starting with 67 CAP only cars, and 68-69 is when the front went hidden and the rear had a lock screw to prevent over adjustment. 70 they went to the plastic cap on the mixture screw to limit adjustment. Must have been a Chrysler method for limiting, as other manufacturer Carter Carbs did not adopt this method. It only had a short run with Chrysler too.

As far as the screws, if they are long enough, you can use a Dremel blade and cut the slot longer. Then spread it some. This will act as a a self locking method to tighten up the idle screws. Or you can use the 30A-80 idle mixture screw which the 66-67 hemi used (and other period AFBs) and is similar to the hidden screw on later carbs. Even the AVS idle mixture screw version is very close. They are #10-32 screws at least for the 66-69 front. I have not actually seen a 70 screw to know for sure, but I assume it is probably the same.

What is your carb Number?