Overland Cruisers of Bozeman MT has developed a kit to fit the Toyota 4.7 V8 and a H55 transmission in a FJ40. This kit gives your FJ40 Toyota V8 performance.
There is no reason to have matching spares. Just carry appropriate spares, you still still need to carry two anyway.
16" LT tires are for one ton vehicles and will not air down enough to provide floatation.
I can't see how the there would be an issue with the GPS if you never have had the radio out. There are many features on the map screen that make things confusing.
This is the post I made about changing out the XM antenna to an external high gain antenna.
https://forum.ih8mud.com/threads/adding-an-aftermarket-xm-antenna-to-a-200-series.1254947/