I am trying to find out what i need to to put a 302 or 351 into my 87 Cougar with a 3.8 v6.
You'll need the uber rare engine mount brackets for an '86-'88 Fox Tbird/Cougar. (Or order Chuck W's custom made mounts. Best option)
The proper year engine harness.
Obviously, the engine, and all necessary parts.
The fuel lines are different, in regards to 3.8 vs 5.0 cars, in regards to how they enter the engine bay.
If going with a 351w, you'll also need the Windsor/Fox swap pan and pickup.
These are the basics...it gets deeper when you get to the electronics...staying efi, or going back to the stone age with a carburetor?
if efi, you'll need at least a 5.0HO equivalent fuel pump, the required EEC, and of course the aforementioned engine harness.
I wouldn't bother with putting a carb 5.0 in....you can swap in a Mustang 5.0HO and make more power, more reliably, with less hassle.
A 351w will cost a good bit more to keep efi, but a few guys here have done that on a budget and come out with pretty good power. Search 1Sick88Tbird's posts on his 5.8 swap...
check CoolCats (http://"http://www.coolcats.net/tech/advanced/") for more, detailed info...