Necessary Mods To Achieve 400hp?

At least the OP is doing one very important thing the right way... Buying a stock 5.0 and building it himself. It forces him to learn about the car and it keeps him from having to troubleshoot other people's mechanical/electrical mistakes. Some guys out there just do not know how to do even basic mods the right way. They take a lot of shortcuts. Then you pay for it later. I bought my first mustang when I was 16. Bone stock 89 GT with 160k on the clock. Beautiful car. Drove the hell out of it for a year. Never broke anything. Decided I wanted to go faster but knew very little about cars. Sold mine and bought a supercharged 89 GT with all the bolt one for 1,500 more than I sold my previous car for.

Killed the clutch in two days. Fixed that with a friend's guidance. Killed the T5 a week later. Upgraded to a tko, which then needed a new, new clutch. drove it for maybe a month, then constantly had problems for another year. Never had it running right, spent tons of money, and couldn't find the gremlins in the car. Gave up on the gremlins and bought a new motor. When I tore the old one down, found the face of a piston was completely blown off. Inexperienced, I had no idea. Then quit my high paying job. Had no money. Got the car together barely, but not tuned with the new motor. Limped it around the block one time. Then parted it out for pennies on the dollar.

Building my new one out the right way ten years later. Feel much better this time around.

I still really enjoy it when I drive mine, it's just not convenient or practical in most situations, especially because the fiancé doesn't like riding in it because it gives her a headache....

Glad to hear that you are interested in a Foxbody instead of some import like someone else had posted. I bought my Foxbody when I was 18, fixed it up and got it on the road just around my 19th birthday. Ten years later I must say I am much wiser and respect the car a lot more now than I did when I first had it. Doing modifications is a lot of fun; I drove mine around that entire summer in stock form and started ripping it apart the following winter and have been hooked since. With that being said, my advice is to get a feel for the car and enjoy it before tearing into it. You will be surprised to find out that these cars feel pretty quick even stock, especially if you haven't had any experience driving something like that before. The first modifications I would do are getting torque boxes reinforced and full length subframe connectors welded on. That will protect the unibody structure of the car when beating on it. I didn't do mine until about 3 years after I bought it and modded it and wished I would have done it way sooner. Welcome and Enjoy!