Well, before you get too excited, the Mustang motor and the Tbird motor are one and the same. Don't forget that the 5.0 has less power than the SC, stock for stock. The only difference in output from the LX Tbird and the Mustang is attributable to the quieter exhaust system.
One of the main reasons that the swap is difficult is that all of the dash functions are integrated into the wiring harness that goes to the computer. For example tach wire, gauge wires, abs wires, vmm wires, a/c wires, cruise control wires, and more. So when you pull the SC computer harness out, a lot of your electronic functions go with it. It is a major chore to sort it all out and get it to work right.
As for putting the m90 on the 5.0, there isn't any kit out there that contains the brackets and pulleys to make it work. You would also need a totally new intake manifold. Being that the M90 is not going to add a whole huge chunk of HP to the 5.0 due to its small size, I would recomend looking for a Kenne Bell kit made for a Mustang and start there. Otherwise it is just not worth the effort and engineering that would be required. If you really don't want to mod the V-6 for more power, then I suggest you look towards a different car.
Putting in a motor that functions independantly of the SC's computer system would be an alternative like what Prostreet Rich has done, but that means ditching the EEC and running either a carb or an aftermarket EFI system, in which case I'd look at either a 351W or something even bigger.