I worked on the hangers for a number of years on production control, including on the Vanguard conversion, re Merchantman floor height - yes they were lowered to accommodate the size of the std containers then being used, we also had to modify the fuse structure to take the door frames and additional loading. Each conversion lasted some weeks, it was a full stripdown.
The question about engine noise, there was a problem with sync, we could never get full snchronisation with all four, from memory it was No 2 that was always slightly out giving a pulsing throb through the airframe. As an aside, on each check we tested the wing structure flex, jacking each wing tip up 13 foot, we never did work out how to check downward flex.
Cheers