yes a 1990 2wd will have a L6 as standard.

yes that's possible, but he'll also need a 4x4 TPS known as a PF09 (also used on some cars from the Fix It Again Tony group), and you'll need to swop the outer 2 wires on the tps sensor connector around as the PF01 and PF09 work the other way around.

get the fuelling checked afterwards as well.