You do realize they went to the 50 Cals for a reason right? Be careful what you wish for 
The dual .50's came about late in the game, ultimately LW. The Lancaster was available early in 1942 and ultimately stayed the same with exception to the guns and minor engine tweaks. I'm under the impression that only a minority of Lancasters were fitted with the dual .50's in the tail. I just read in one of my books that the Lanc Mk III was ordered with the quad .30's, then part of the way through production the dual .50's were installed, but the decision was made to go back to the quad .30's due to logistics: there were already more than enough quad 30's turrets and mounts made and the dual .50 turrets and mounts had to catch up in production.... at least that is my interpretation of the information in a nutshell. Plus, I'd have to think that as the war went on there were less and less German fighters to contend with.
Less than 200 of the 7300 built had the dual .50's in the rear turret??? My source ("The Encyclopedia of WWII Aircraft", Editor: Paul Eden), is a bit vague on those numbers too.