I pretty much know where the R179's will be once they go into service. The 10 car sets will all go to the A, and the 8 car sets on the C, and J/Z. According to Wikipedia, at peak levels of service, there are currently 64 R32's on the C, 80 on the J/Z, 40 R42's on the J/Z, and 20 R32's on the A. Since there are 274 R32's and R42's combined and 204 needed for peak service levels, there are 70 extra cars. There are 300 R179's coming in with 40 in 5 car sets and going to the A. The other 260 will be going to the lines that need 4 car sets. There are only 184 of the old cars needed during peak levels of service, so the fleet of 260 should be more then enough to replace them all.
Here's where it gets confusing. MTA says they plan to keep a small number of R32's around until the R211's arrive due to fleet expansion needs and service changes relating to the L train shutdown. As of now, they've said they plan to max out J/Z, and M service and run full length trains on the G. Some of the L's fleet can run on the J/Z and M since it's route will be shorter and might not need as many cars. The G will need to more than double it's fleet to run more and longer trains. My guess is that they will put the R32's on the B, so that they can move some of the R68's from there to the G. They'll be wanting to minimize use of the R32 fleet and the B's fleet gets weekends off. They want to keep them on above ground routes, and the G is almost fully underground while the B is above ground for most of the run in Brooklyn. Since the R211's will be in all 5 car sets (10 car trains), they'll likely want to have the remaining R32's running on lines that require 10 car trains so that they can directly replace them and not have to move fleets around much.