The two screws on the front of the baseplate are just the idle mixture screws. Adjusting these will not effect the part throttle/full throttle mixture. That is done by the meetering rods for the primary and secondary side (two pair). If its just the part throttle/cruise mode that is running rich you should be able to work with running a larger rod on the primary side.
The problem is, Q-jet rods/jets/hangers are not easy to come by only Edelbrock has any of that stuff now and it's pricy. You might be better off to run down to the local boneyard and pull the lid's off of all the q-jets in the yard (if they still have the engines) and take the rods, jets and hangers. I did this a few years back and put together a good selection. Then go get the Rochester Quadrajet book published by HP Books. that book has an excellent guide to setting up a Qjet by jet size, rod size and hanger length.
That being said don't overlook the basics. The float level may be set to high, causing the rich condition. Another issue common to Qjets is too much idle speed adjustment exposing the transfer slot pulling fuel from it in addition to the idle circuit. One more issue leading to a rich condition could be from the plugs in the bottom of the main body of the carb. These plugs are just there to fill a hole from the casting process, but are known to leak over time. This leak is just above the throttle baseplate and gets pulled right in with the rest of the fuel from the carb. The fix is to remove the baseplate and mix up some JB weld, clean the plugs and surrounding area on the carb and apply the JB weld to the plugs to seal them up. If those issues are not there, then start looking at the meetering rods.
