Not entirely sure you are correct as the DOO equipement and station lengths wasn't sufficient for 10 Car DOO. 377/3s were taken from the coast to metro (in exchange for 313s) to make the 10car Metro! 2x 377/3 + 1x 377/1-2-4 Because they have SDO and OnBoard DOO. The 376s have neither of these as it stands so, unless these are added they're not an ideal replacement for Metro workings. In that case new trains might be cheaper... Could always be shortened to 4 car to replace 455s.
A quick look at Thornton Heath Station (not my video) shows the DOO issues where the platforms were extended to 10car from 8.