Fortunately there is performance data available for the Lancaster VI. The Lancaster VI is basically a standard Lancaster fitted with two stage Merlin engines. With these engines the Lancaster's altitude performance and speed at high altitude increases considerably.
The improvements do not come up to B-29 standards.
Lancaster Performance Trials
http://www.wwiiaircraftperformance.org/Lancaster/jb675-level-speeds.jpg
http://www.wwiiaircraftperformance.org/Lancaster/jb675-climb.jpg
http://www.wwiiaircraftperformance.org/Lancaster/Lancaster_VI_JB675_Performance.pdf
The Lancaster VI used in these tests had all 3 turrets and H2S radar fitted.
Lancaster VI speed at 24000ft is 325mph. This is the maximum speed. At 25000ft speed drops of to 320mph. The Lancaster VI service ceiling, where climb drops to 100 feet per minute, is 28000 feet. The operational ceiling, the altitude at which climb rate drops to 500 feet per minute, is 23500. Thus operation above 24000 to 25000ft is impractical.
By Contrast the B29A had an operational ceiling of 30,000ft instead of 23500 and its speed in the 25000ft to 30000ft altitude is about 357mph. (Data below)
Hence the Lancaster VI would be attacking at 83% of the altitude which increases blast by the inverse cube power (1/0.83)^3 which is 50% more blast.
The fall time of the bomb before detonation is about 10% less and the speed of the aircraft is also 10% less in addition the air pressure is more. All of this adds up to the factor of 2 safety margin for blast being reduced to 1.
As far as I can tell early British plans for grand slam bombs and nuclear bombs go back to the Vickers Windsor.
Using a Lancaster requires the yield of the bomb to be dropped 50% or the use of a parachute retarded weapon to maintain safety margins.
A small increase in speed and operational ceiling will occur through deletion of the upper turret.
Boeing B-29A Superfortress
Engines: Four Wright R-3350-57 Duplex Cyclone eighteen-cylinder air-cooled radial engines each with two General Electric turbosuperchargers, delivering 2200 hp for take-off and having a war emergency rating of 2300 hp at 25,000 feet. Performance: Maximum speed 357 mph at 30,000 feet, 306 mph at sea level. Maximum continuous cruising speed 342 mph at 30,000 feet. Economical cruising speed 220 mph at 25,000 feet. Initial climb rate 900 feet per minute at combat weight. An altitude of 20,000 feet could be attained in 38 minutes. Service ceiling 33,600 feet. Maximum range was 4000 miles at 25,000 feet with 5000 pound bomb load. Practical operational radius was 1800 miles. Maximum ferry range was 6000 miles. Weights: 71,360 pounds empty, Normal loaded 120,000 pounds, maximum overload 135,000 pounds with 12,000 pound bombload. Dimensions: wingspan 141 feet 2 3/4 inches, length 99 feet 0 inches, height 27 feet 9 inches, wing area 1738 square feet. Fuel: 8288 US gallons after installation of semi permanent bomb bay tanks. Armament: Twelve 0.50-inch machine guns in four remotely-controlled turrets (two above and two below the fuselage) and in the tail, each with 1000 rounds of ammunition. In addition, early production blocks had a single rearward-firing 20-mm M2 Type B cannon with 100 rounds in the tail position. The 20-mm cannon was deleted on production block 20 and two more 0.50-inch guns were added to the forward top turret.. Maximum internal short-range, low-altitude bomb load was 20,000 pounds. A load of 5000 pounds of bombs could be carried over a 1600-mile radius at high altitude. A load of 12,000 pounds of bombs could be carried over a 1600-mile radius at medium altitude.