Modified or new K-member. 3.8 V6 k-member is the same as 5.0 also any 85 - 95 5.0 K-member will work as well.
Brakes system will need to be replaced. 4.6 uses hydrastat off the PS pump. 302 will need a vacuum system minus all the ABS.
Will need a different transmission,.. T5 or early AOD or C4. Will need a gauge cluster from a 94/95 car or buy the autometer cluster and make your own gauge cluster. It will be a wiring nightmare from hell to get everything working correctly. You won't need to change the transmission if you get a Trans controller that will control transmission separately and run a TPS on the carb very simple and The quick 1 controller is great for the money!
Much easier to buy a 94/95 or start out with a V6 car instead of a 4.6 car.
Here's you a good write up to read about this.
http://www.allfordmustangs.com/forums/v6-tech/287072-v6-v8-swap-faq-newbies-read-here-first.html
Hope this clears things up for y'all! Take care !