Some Chevy carburetors were set up with huge lower main airbleeds, and restrictions in the uppers.
A 77 main jet is HUGE if used with a "K" metering rod, and it will be pretty rich at heavy/full throttle.
If you can't tune within an acceptable range with the APT using the 71 main jets, move up to 72's, not clear to 77's.
Also keep in mind that the float level, fuel inlet seat diameter, fuel pressure, etc, effect the A/F ratio some.
In many cases, one can simply raise the float level slightly and add enough fuel to get within the range that APT will eliminate the lean condition.
In any case, make small changes when tuning, just one jet size is usually enough.....Cliff