we have done a couple of latemodel drivetrains into the classic ponies. Depending on how involved you want to get with it and what your ultimate goal is. 05 or newer v6 mustang
drivetrain would be very fun and more powerful then the standard v8 originally in the classic pony.You can find these on the internet fairly reasonable. If you are wanting something a little less involved and for a few less dollars, look into buying a donor v6 mustang from the sn95 era.You will still have a pretty fun car with decent mileage.
On a side note,you can build a 5.0 car that will get 20 to 23 mpg with a carb. and still maintain great reliability.One of our first project cars was a carbed 91 roller motor that averaged 21 mpg with a t5 and 327 gears.