The Fox body Mustangs were light but over the years with changing options and federal regulations the weight went up. Federal regulations is the main resound the s550 weights so much.
Mustang isn't going to get a 6.8L engine unless one of to things happen. One the engine goes into the F150 our two it's s specialty engine for a future High Performance Mustang. Everyone needs to remember that Mustang wouldn't have a V8 now if it wasn't in the F150.