I converted my I-6 65 convert to 5.0. Yes, it was/is a lot of work (almost finished), but a major upgrade in the car's performance. The 5.0 is the same block as the 289, so everything fits - the T-5 transmission shifter even comes up through the floor in the same place!!! (Use 66 motor mounts) The computer and wiring can be a bit pricy (the conversion engine wiring harness alone was almost $500 from Ford), but I bought the engine, trans, computer and original harness in a package from one donor car and that helped. Be sure to get all the sensors with the engine, especially the barometric pressure sensor.
It is a lot of work but worth it, I think. Just depends on what you want in a car. Mustangs Plus in California has a lot of the pieces you need and some good instructions on particulars.