Brian,
I've run EFI on my flathead for several years now. I can tell you that you likely can run quite lean during cruise with no problems. I run around 15:1. I target 12:8 under full throttle, which seems to work best for my set up. It likes to idle at around 14:1 or so.
Of course, tuning is dependent on your particulars (engine specs, car weight, transmission, gearing, ect). All these factors play a role in your tuning. I'm running a mild cam, electronic ignition, and 5 speed trans in a light weight rpu.
Something else to consider, tuning is much easier with EFI. I make tuning changes while driving and can feel the immediate effects. This makes it much easier to determine what the engine wants, something you can't do with carbs. But, tuning a carb is still the same. Make a change and see what it does. JWL was right on in his post above:
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If "under load" means wide open throttle and if the timing is not slow the main jets might need reduced and PV might then need slightly upsized.
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At least you have a wideband. Its absolutely the best tool to use for tuning. No more guessing. It will tell you if your changes work. I don't know if you have the ability with your wideband, but if you do, datalogs are your friend.