Production was the reason Merlin Spitfires continued, not range.
The Merlin was being built in several factories in the UK plus a couple in the US (the factory built for the Continental IV-1430 was eventually put towards Merlin production, but not many were built there). In contrast, the Griffon seems to have only been built at Derby and Crewe, Rolls-Royce factories.
Plus there was additional fuselage strengthening required for the XII, XIV, etc. This would have required some sacrifice of production to change over, as well as the new cowlings required.
The fuel capacities of the IX, XII and VIII were as follows:
IX: 85UKG
XII: 85UKG
VIII: 124UKG
In addition to extra tanks in the wing, the VIII had a larger upper tank.