Hi,
Depending on the ECU (PRC8747 does not need tuning resistors, but PRC 7081 does)
the 3.9 efi can work without lambda sensors (called open loop), but you need to change the tuning resistor to the "green" 470 ohms.
the tuning resistor is in the loom somewhere near the ecu connector it is 2 wires and a connector, with 2 wires going into a flat clearish plastic sleeve with a resistor inside.
I think the cat ones (with lambda sensors) are white or some other colour.
I believe there are 4 ranges of resistor
3900 ohms white,European catlyst
910 ohms yellow, sudi non cat
470 ohms green, uk and eu non cat,
180 ohms red, australia and rest of world
Cheers
Stephen