Some of the modifications don't seem that difficult.
Which ones? As so far it comes across as all too hard
But the whole rear tank set up seems a bit dodgy compared to the Mustang.
If you changed that to had more problems you would join everyone else.
Mustangs in US service were allowed to keep using their rear tanks for years after the war (I don't know if the shifted anything else. like radios or got ballast or????) High back Spitfires were allowed to fill the rear tanks with special permission, otherwise wired shut. Low back Spitfires were prohibited from using the rear tanks under any circumstances.
As has been reported many times, add the RAF was wary about operating wartime Mustangs using the rear tank. The post war pilot's notes for the Mustang say carrying under wing loads made the handling better and caution about the effects of dropping the loads with the rear tank still full. So putting a 90 gallon external tank on the Spitfire should to the same sort of thing. As the cut down fuselages on the P-47 and P-51 caused handling issues, requiring a dorsal fin extension, you would expect the same for the Spitfire.
The test of the Spitfire in Jan 1945 used metal covered elevators. A relatively simple change but not done on service aircraft even post war? This is the plane with evaluation of burning down the fuel by 30 gals leaving 43 in the tank/s.
Actually wrong, the early 1945 tests used a standard Spitfire that was also given hand made metal covered elevators for *some* of the tests, they made handling better. There were July 1944 tests on a mark VIII with a 75 gallon rear fuselage tank. The 30 odd gallons level had been worked out some time previously. Not sure if the 1945 tests made it official or not. The post war Spitfire IX and XVI notes state all XVI and some late model IX had the rear fuselage tanks, the effective start of XVI production was September 1944.
Post war Manual says 30 gallons remaining was dividing line between what you could do and what you couldn't. Extra insurance in peacetime or different elevator required a different limit or both.
Whatever. It is clear you consider the rear tank in a Spitfire a bad idea, you are just having trouble finding a good reason why, trialing various thoughts like reaching back to 1942. The US mark IX modification was adding 43 gallons in the rear fuselage, plus 16.5 gallons in each wing and plumbing the wings to allow a pair of 62.5 gallon drop tanks, similar to the Mustang. The Spitfire VIII pilot's notes have a 26 gallon rear fuselage tank
Low back Spits (at least MK XVIs ) might have benefited from a broader tail/rudder like the Griffon powered XIV. when trying to fly with a rearward CG?
As a first problem that adds weight aft, which makes the CoG issues worse. A post war trial with a 58% bigger tail on the mark IX had a mixture of handling advantages and disadvantages.
US radius charts were done at 25,000ft. The advocates of the Spit want to use 20,000ft. How do you want to cover the difference?
The one data point we have for calculating the Spitfire is at 20,000 feet and "advocates" is an interesting adjective, does that make you a detractor, anti advocate? As for the altitude correction the post war Mustang notes on fuel consumption with altitude have the datum point as 10,000 feet, and say consumption increases over datum by 4.5% at 20,000 feet and 5.5% at 25,000 feet.
The US also figured the exit speed at 215 IAS. I am not saying they were right but we do need to use a common speed to compare planes.
So the P-38 and P-47 combat radius figures require them to use the same speed as the Mustang? Remembering that being slower than the Mustang they would have the same reserve power issues you brought up about the Spitfire.
We have no idea how the 8th Air Force calculated combat radius, how much allowance for formation flying, for the inevitable difference between aircraft and pilots, whether it was a fast climb to operational height or a steady one, the climb fuel allowance, if economic cruise was employed between base and a given distance from hostile airspace or further and probably other factors. Until that is clarified no one can apply similar adjustments to the Spitfire.
According to Roger Freeman adding an 84 gallon external tank gave the P-47 a combat radius of 280 miles, 108 gallons external increased the radius to 325 miles, so around 90 more miles at a mile a gallon. Upping the external load to 165 gallons increased radius to 375 miles, so 190 more miles from 81 gallons, over 2 miles per gallon. The different external loads 165-108 = 57 gallons for 100 more miles. At 2 miles per gallon going out and back to 280 miles requires 280 gallons, leaving 305-280+84 = 109 for combat and reserves for the earlier P-47D, going out to 325 miles leaves 108-20 = 88 for combat and reserves, to 375 miles leaves 165-70 = 95 gallons for combat and reserves.
According to Francis Dean the yardstick range for a P-47 with 305 gallons was pushing 1,000 miles, so over 3 miles per gallon but adding another 65 gallons internal does not raise the range by around 200 miles. His combat radius with 370 gallons internal is 225 miles, adding 300 gallons of external fuel pushes that to 670 miles, so over 3 miles per gallon. Mission profile is 5 minutes warm up and take off climb to 25,000 feet covering no horizontal distance, cruise (but not at what setting), drop tanks 5 minutes combat and 15 minutes military power for combat, 30 minutes reserves at economic cruise.
Essentially keeping the Spitfire VIII range comparable to the P-47D was possible for much of 1943 as built and required around 33 gallons of rear fuselage fuel to remain so in early 1944, and the engineering issues for the additional tank were historically largely solved in 1942/43 given things like the rear fuselage ferry tank for the mark V and we know that the issues were not major for that fuel load.
Arthur Harris turned down an offer from James Doolittle to mount a combined day mission to Berlin in 1944 citing the lack of fighters available. Eaker had the 1943 problem of a lack of fighter range but also a lack of fighters compared with his bomber strength, it meant running unescorted raids or an underemployed bomber force.