In the book about the Bristol Brabazon there are numerous photos. Some of them are of the inside of the huge hanger that was built for the Brabazon at Filton. There's two (or three) which show a B-29 that the RAF got, going through their conversion to RAF standard. Although it doesn't come right out and say it, but I got the impression Bristol had the maintenance job for them too.
I had this thought of a B-29 which had arrived for some damage repairs, like after a heavy landing, and the first thought was to scrap it. But then the Bristol engineers, suggest they repair it and upgrade it at the same time. My thoughts were to put Britannia engines and wings on it ----- along with the 4-wheel u/c ---- My thinking was the wing got a new build fuselage section, and then the front and rear fuselage parts of the B-29 were added to this new center section, making it longer. To finish the job, a B-50 tail was installed.