For the transmission I'd use Valvoline MaxLife in the red bottle and a tube of Lubegard ATF additive. IIRC the capacity with a converter drain is around 8 quarts.

For the rear diff, 85-90 if it is a cold climate. If it is warm or a high mileage car I'd use 75-140.

For the late shifting, have you tried loosening the detent cable 1-2 turns. How late is it shifting? 1-2 is a slightly prolonged shift on those transmission.

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