I have a 1986 LeSabre Custom Coupe with a 3.8 from a 1990 park avenue.
I wanted to get as much power as possible and was wondering if there
was a certain super charger that i needed.

Well of course, the easiest would be the Eaton supercharger that is on the Ultra's, but there's more involved than just dropping that on your existing engine.

first of all you will have to upgrade the computer cause with out that you stock one wont know what to do with the it and after that i would get it dyno tuned

if you put a supercharger on it, when you get it dyno''d do a chassis dyno.

whats the difference?

Sell it! Buy something with 8 cylinders and rear wheel drive! I''ve always loved little motors making big power,but what your talking aboutis an expensive prospect with out much gain. Who told you that a LeSabre is even SUPPOSED to go fast? If you really love that car, drive it like it is. If you really need to drive a supercharged V-6, find a car that was born that way. You''ll save a fortune and a lot of walking home!