(I'm a 4.9 guy) The 3.4 will be a slightly simpler and cheaper swap, but there are great resources and support for both. You need to decide what you plan on doing with the car to decide what will make you happy. You can't go wrong with either, but if you really want power and all out performance, then you are better off starting with the 3800sc. Be aware that the supercharger sound is not for everyone...you'll be listening to it a lot, so you should do a search for some 3800sc videos (taken from within the car) to get a feel for what it sounds like.
There are issues with the 84 that need to be dealt with. Sounds like you are already doing a harness swap. I'm not sure what else, if anything may be required.
Good luck! ...and keep the thread going!