To do the conversion from the v6 to the 5.0L there are going to be some major alterations needed. Like the previous note the best way to go is to purchase a donor car. Now if your not completely set on dropping the 5.0L in their you might want to consider a 4.6. The 4.6 bolts up the same as the 3.8 and there are fewer alterations needed. Either way they will both take time and it might take a few weekends. Checkout www.stangparts.com they have some decent prices on used engines and you might be able to get a good deal on parts.
Yeah i considered to swap my V-6 with a 302 but discovered that swapping with a 4.6L would be a little easier with my 1999 Mustang. Even though a 3.8L is pretty much a 5.0L minus 2 cylinders. Its a costly and timely project. So far i have the old motor out and getting a dressed, long block saleen motor in a couple weeks. -william arthur