I looked into it with my 3.8 2000 Mustang. A trusted mechanic and I did the math and you will end up spending lots of cash ($4000 +) for the swap. Note that the computer, wiring harness, transmission, motor mounts will all need to be changed as well as the engine. It truly makes more sense to just upgrade to the 8 cylinder car - as a whole. Good Luck !
You can buy a DOHC enigne off ebay out of a mercury marauder which is the exact same engine that is ran in the 2003 Mach 1s. That will run you about 400-700. It should come with the wiring harness and computer if you shop smart. The tranny is going to be about a 1000 for a good t-45 and that is the only one to have next to the t-56. Doing the swap is not that hard if pull the old out and drop the new in. Solid motor mounts are $80 which give the car bout 3 hp b/c there is no motor torque but you will have slight slight vibration from about 650 rpm to 1100 not that bad.