[ERA Chas]

SuperHart's advice is correct-the rear carb must have the linkage bias to come on in staggered relationship to front. For years I ran it with the slotted link opening the rear carb after about 10% of primary.
Then I read a tip here from other members who had better response syncing them 1:1. You have sharp eyes seeing my new link. However this will only work best with a lot of cam and 300+cfm heads.
I advise you set yours up correctly with the bias in the linkage. Your motor won't support much modification. You will have lousy street performance if you don't.
Actually the advice by SuperHart and Saltshaker to have a good shop set up your 390's is very practical and they would have great throttle response on your type of motor. You're not going to find 40HP-you want to maximize low-end torque which is your motor's strong point.