I imagine after a few years, likely during the next refresh, they will drop the 3.7 completely.
These numbers are a bit underwhelming to be honest.
The 2015 Mustang line up should be:
Base model - Turbo 4 - 310 hp / 320 tq - $23k - $28k
Mid Range - Turbo 6 - 365 hp / 350 tq - $26k - $34k
GT - V8 5.0 - 450+ hp / 400+ tq - $32k - $40k
Special model - V8 6.2L NA - 500+hp / 460+tq (for Boss/Mach 1/GT350 etc..) - $46k - $52k
High end - FI V8 - 700+ hp / 650+tq (Shelby or Cobra) - $58k - $70k
That would cover every kind of budget, while allowing everyone to get the Mustang they want. Five engines may seem like a lot but consider the current F150.. You have the 3.7, 5.0, Ecoboost 6, and 6.2. Just one more engine for a high end supercar level Mustang, which already exists (Trinity 5.8). The transmissions between the EB-4, 3.7, 5.0, 6.2 and EB-6 are all the same, just need the transmission calibrated for the autos and each engine, give the manuals the same transmission and adjust the final drive gears for each combination. It seems like a lot of work, but it really isn't.
If this new Mustang doesn't perform exceptionally well, which judging from the numbers it may not, GM's next Camaro is probably going to stomp a mud hole in these ponies' asses and kick it dry.