If you want to go carburated it might be a little easier, if you want fuel injection, then you'll have to get a computer and wiring harness to start. You'll also need to change the front suspension in order to handle the extra weight of the 4 extra cylinders. Oh yeah, and think about upgrading the radiator, maybe also get a fan with the shroud out of a Thunderbird, I believe, (but don't quote me on it) that the fan from the Mark VIII will also fit. The rear end is a must if you want to have any traction. My friend did a V-8 swap into an 89 LX, and had the 7.5 rear end for a little while......With 650 hp....
Needless to say he had some traction problems.