Yeah, apart from perhaps the increased fuel economy...i can't think of any reason to go with an engine that will require custom work to even fit/operate--and then offer less power (if i'm reading those stats correctly).

Personally, I would probably go with a roof-rack & some 5 gal cans over swapping out the engine...but that's just me.

KBB has NO idea when it comes to vehicles that are nearing collector car status. There are certainly some that are worth 8k. there are others that would be overpaying at 4k. These are so old now that there's a LOT of variation between examples--because of multiple owners.

As far as sliders/brush guard--plenty have done it. Jo's is probably the gold-standard, imho. Keep in mind that any change to the rear end gear ratio is going to worsen fuel economy--but it is quite popular of a swap.