How many miles are on your engine? Generally speaking, a rebuilt engine is only going to get you back to the original power level that the vehicle came from the factory with, so unless you have very high miles or a lot of engine wear a rebuild won't get you much in terms of additional power. Making sure that your maintenance is up to date (quality Iridium spark plugs, quality plug wires, filters, etc,) and using a synthetic engine oil is more likely to get your engine making all the power it's capable of.

I don't know that anyone has found any simple ways to extract more power from the 3.5 SOHC. The engine is designed pretty well from the factory and they didn't leave much on the table as far as easy performance improvements. Some small gains could be had by going with the maximum overbore pistons on a rebuild, a slight increase in compression from milling the heads, and possibly some airflow gains through match porting the manifolds to the heads, smoothing any imperfections in the ports, and a quality valve job, but you would really need to do your homework beforehand so you don't create any additional issues.

Engine swaps are only limited by how much time or money you want to spend on it. If you have the tools and know-how to do it yourself then the sky's the limit, but more often than not an engine swap is never completed properly and ends up being sold for parts.

In my mind your best option would be to change your axle gears to 4.9's from the 4.2's that your Montero was originally equipped with. The gears can be sourced new or you can hunt the wrecking yards for them - they will restore more acceleration than what you lost with the larger tires that you have.