Avro Shackleton
General characteristics
Crew: 10
Length: 87 ft 4 in (26.61 m)
Wingspan: 120 ft (36.58 m)
Height: 17 ft 6 in (5.33 m)
Wing area: 1,421 ft² (132 m²)
Airfoil: modified NACA 23018 at root, NACA 23012 at wingtip [15]
Empty weight: 51,400 lb (23,300 kg)
Max. takeoff weight: 86,000 lb (39,000 kg
Fuel capacity: 4,258 imperial gallons (19,360 L)
Powerplant: 4 × Rolls-Royce Griffon 57 liquid-cooled V12 engine, 1,960 hp (1,460 kW) each
Propellers: contra-rotating propeller, 2 per engine
Propeller diameter: 13 ft (4 m)
Performance
Maximum speed: 260 kn (300 mph, 480 km/h)
Range: 1,950 nmi (2,250 mi, 3,620 km)
Endurance: 14.6 hours
Service ceiling: 20,200 ft (6,200 m)
Max. wing loading: 61 lb/ft² (300 kg/m²)
Minimum power/mass: 91 hp/lb (150 W/kg)
Armament
Guns: 2 × 20 mm Hispano Mark V cannon in the nose
Bombs: 10,000 lb (4,536 kg) of bombs, torpedoes, mines, or conventional or nuclear depth charges, such as the Mk 101 Lulu
Avro Lancaster
General characteristics
Crew: 7: pilot, flight engineer, navigator, bomb aimer/nose gunner, wireless operator, mid-upper and rear gunners
Length: 69 ft 4 in (21.11 m)
Wingspan: 102 ft 0 in (31.09 m)
Height: 20 ft 6 in (6.25 m)
Wing area: 1,297 sq ft (120.5 m²)
Empty weight: 36,457 lb (16,571 kg)
Loaded weight: 68,000 lb (30,909 kg) [42]
Max. takeoff weight: 72,000 lb (32,727 kg) with 22,000 (10,000 kg) bomb
Powerplant: 4 × Rolls-Royce Merlin XX liquid-cooled V12 engines, 1,280 hp (954 kW) each
Performance
Maximum speed: 282 mph at 63,000 lb (28,576 kg) and 13,000 ft ( 3,962 m) altitude [37] (246 knots, 455.6 km/h)
Cruise speed: 200 mph (174 knots, 322 km/h)
Range: 2,530 mi (2,200 nmi, 4,073 km)
Service ceiling: 21,400 ft at 63,000 lb (32,659 kg)[37] (6523 m)
Rate of climb: 720 ft/min at 63,000 lb (28,576 kg) and 9200ft (2,804 m) altitude[37] (3.66 m/s)
Armament
Guns: 8× 0.303 in (7.7 mm) Browning machine guns in nose, dorsal and tail turrets, with variations[42]
Bombs: Maximum normal bomb load of 14,000 lb (6,300 kg) or 22,000 lb (10,000 kg) Grand Slam with modifications to bomb bay.[42]
The Shackleton is a bigger aircraft, and is quite a bit heavier. But it is (marginally) faster.
I would think that swapping the Merlins for Griffons could be done relatively easily. The problem then would be that the range is reduced because of the higher fuel consumption of the Grifons. That would need some added tankage. I suggested that wing tip tanks could be used.
The next thing is whether the extra weight of the engines and fuel reduces the bomb load to be carried.
A Griffon VI is about 400lb heavier than a Merlin 20 series. Call it 1600lb all up. If that has to come from the bomb load, is it worth it?