""I would not expect the secondary HSABs to have any effect on the 50MPH cruise AFR since I can't imagine that the secondaries are operating on the main circuit at that point.""

You are EXACLTY correct. I swapped the rear .039" HSAB's for .036" HSAB's and went for a drive. No other changes and the outside temp was 61 degrees vs 59 degrees yesterday.
Yesterday I showed my 50mph to WOT as 12.8 (D) and 12.8 (P)sides. Today I tested this three times and found 12.6/12.6, 12.6/12.4,& another 12.6/12.4. I have hit the 12.5 mark!! The car felt strong and I am happy with this.
My 40mph, 50mph, and my 60mph cruise A/F numbers WERE NOT AFFECTED by the secondary HSAB swap.
I had the exact same numbers as yesterday on cruise!!

Thank you Dave!!!!!

Note: My float levels are right at the mid line on the bowls. This allows me some tweaking to lean it out. Since I drive in 50-95 degree weather, this gives me some room on both the main and the secondary float adjustments for leaning it out in warmer temps. Now that I know that I can gain/lose .2-.4 on the secondary WOT reading with the HSAB swap, this is another external (quick) change that I can make during a normal driving year.
The metering screws handle the idle to off idle A/F changes and the IAB's handle the transition circtuit changes, I need to test to see if my existing internal carb setup is good. I am at 63's for front jets, .039" HSAB's up front, 9.5 HFPV/.071 PVCR's, .065 IAB's (all), .033" IFR's (all), 78 rear drivers and 76 rear passenger jets with the .036" HSAB's in the rear.
Time will tell.