I am having trouble getting my Thermoquad equipped 440 to give a consistent AFR across a range of cruising speeds and am hoping for some Carb guru help!!

I recently changed from a Carter AFB to a rebuilt Thermoquad on my mildly tuned 440 engine, partly to improve fuel consumption as well as the increased cfm.
I have stripped the carb and corrected the many issues it had (linkages and float level) and generally the car runs very well. Idle is perfect and responsive to adjusting screws, throttle tip-in is now good after correcting the accel pump and WOT is fine. The problem is that at steady state cruise the AFR varies from 13 at 40mph to 15 at 80mph, I'd like to be able to get closer to 14.5 throughout the range.Secondaries do not open throughout this range.

I have tried different metering rods (original and some I have made) with the only effect being a gross shift in AFR. I have tried different float levels with the same result. Similarly the metering rod screw only moves the issue around and I can dial in an AFR for a single speed only. I am thinking the issue could be bleed hole sizes? or maybe the taper on the rods(unlikely?), but i have no information what the bleeds should be or what to try and alter.

Other information;
- 18 in hg at idle
- 3200rpm at 80mph
- Timing 12BTDC static - standard distributor 36 total
- "mildly polished/ported" heads, Edelbrock performer intake and headers.

Look forward to any suggestions.