You will need more than just the motor and ECU. It's a very involved swap. Your best bet is to get the whole car as a donor, so you can pick up all the parts you'll need.
I'd take motor on the K member with front suspension and brakes, trans, under hood harness and dash harness, dash cluster, ECU. If you haven't done it already, you'll also want to address your rear axle. No point in making power if it all goes to one wheel.
If you just want 320 hp, look at adding a supercharger to your stock 3.8L. Less work, less hassle, usually less money and the same power as swapping in a 4V. Also less weight on the nose.
1998 Roush: M112'd Aluminum block 4.6L 2V 6 speed track car
1999 SVT: Mostly stockish
1970 Mach 1: Resto-modded monster
2000 Roush #5004 - World's FIRST M112'd V6 Mustang. Sold, but not forgotten.