When I bought the 4.2 engine I made sure that it came with all the ancillaries including the the loom. Unless using an aftermarket ECU it is essential. The loom connects the ECU to all of the EFI sensors and input devices as well as the Injectors.
The loom looks fairly daunting when you first look at it, but essentially a lot of the wires are excess to requirements, but it is really not worth unravelling the entire loom to get the wires required. Therefore the best thing to do is leave the entire loom in place and then only use what's required.
There are three multi plugs on the Loom in addition to all of the sensor plugs. These are the large multi-way connector that actually connects to the ECU. This is the easy one. You simply plug it onto the ECU.
The other two multi-way plugs are a Large Brown one and a large white one. These contain the actual wires that you will need to splice into or connect to if you can find mating sockets.
The wires required to get the EFI system running are shown here, yes it is only 6 wires!
Large Brown Plug –
Large Brown/Green cable to be connected via a 25/30A fuse to main Live (Constant)
White/Purple – Feed to fuel Pump
Purple/White – Battery Live Via 15A Fuse (Feed to Fuel Pump)
White connector –
White/Grey or White/Dark green (Probably) – to Ignition switch live via a 5A fuse.
Orange/black – Earth via a 510 ohm Resistor
Yellow/pink – To speed transducer
These details are correct for the majority of vehicles, however I have been told that Rover seemed to have a few variants of the loom, so you need to verify that the above is correct on your loom before attempting to connect.
So when you have managed to splice the loom in as above, and then connected all of the sensors, injectors, air flow meter, Road speed sensor etc. to the other end, you should be in a position to test the EFI fueling.