(2)450 DP's vs (2)750 DP's

That's interesting. I see it the other way. Idle is about normal (for pre-smog) AFR but by 30 mph its going lean and going leaner. I'd be looking to richen or extend the idle-transition so it doesn't go away so quick.
I aim for mid to high 14s for idle afr, if I saw his ~12.5 numbers I’d be leaning it out.