I did a fair bit of WB tuning on the 3310 on my BBC truck and documented progress along the way here.
Although I didn't try a TSR you might find some of the information useful.
With the amount of valve overlap your engine has the WB is not necessarily accurate at low speeds. The observed 14:1 AFR at idle is probably leaner than true AFR due to oxygen making its way into the exhaust during overlap. As you increase rpm and throttle the dilution effect is reduced. The true AFR may not be getting richer with RPM. The wideband is becoming more accurate. I mention this so you don't spend too much time chasing a number on the wideband. Give the engine what it wants.
At 2750 RPM the IFR still has some influence at very light throttle. I'd guess its insignificant when pulling a grade or accelerating. You were correct to reduce the MAB and increase the IAB to try and lean the cruise AFR in the 2,700 rpm range. You could also try reducing the emulsion bleeds in the primary metering block. These should be 0.028", and you can try 0.026". You might need to bump up the PVCRs to maintain WOT AFR. You might also need a higher # PV to keep from going too lean before the PV opens.
installing a TSJ might also work, but if you lean the top of the TS too much you might experience low speed bucking around town. I think its worth a try though.
After you get the low speed sorted out wire the secondaries shut and check WOT AFR on the primaries only and adjust PVCR to desired AFR. No need to run it to 6,000 rpm. A few short blasts in overdrive should do it. Reenable the secondaries and adjust secondary jetting to achieve desired WOT AFR.
If your 770 does not have the vacuum secondary signal tube, I recommend installing one.
I tap the vacuum passage in the vacuum pod for a set screw with a 0.055" hole and remove the check ball from the vacuum pod. Try a yellow secondary spring.