According to a Car & Driver employee from the Michigan area, a Ford insider has leaked information about the 2015 Mustang S550 engine lineup — information about which he posted on 3.7 Mustang forums. The user claims that the next-gen engine lineup will include 3 options.
2.5-liter four-cylinder @300hp
3.7-liter six-cylinder @305hp+ (but he did say there were talks of upping the power to somewhere near 400hp)
5.0-liter eight-cylinder @475hp+
Also notable is that the insider noted the next generation Mustang looks “very European.”