Where to start

Sounds like a project, just do it bit by bit and you'll have it done in no time.

I suggest you install a tubular K-member at this time. The weight saving is big.
It's also a good time to weld in some sub-frame connectors... the roll cage won't cut it all by itself, especially if it's a bolt in style or not welded to the frame.

If you are not putting in an aftermarket K-member, there's no reason to swap it, they're the same in the 4-cyl & 8-cyl...Brake plumbing on the 4-cyl body will work & the under-dash wiring is the same for the 4 & 8 cars so there's no need to swap out dashes either, just the instrument cluster because of the tachometer. the 4-cyl fuel lines obviously won't work. If the car has subframe connectors on it, you'd be better-off doing a braided fuel system since you won't be able to snake the stock 5.0L lines in without cutting the subframe connectors (found this one out the hard way)...It's a pretty big undertaking, but I managed to do it, so you can too...just take your time & make sure you don't dispose of the donor car until you are finished (I made that mistake too), or you might find yourself searching for/buying parts that you just tossed away.

There's a lot more to the swap than I mentioned here, but I figured you'd just ask when you run into an issue...Enjoy & good luck to ya!!