Re: 1990 Fairlane 3.9Litre
No you wont need to buy an aftermarket ECU. Those one-piece ECUs came out with the ED, and some very late EBs as well. The NC I took my loom out of had a one-piece.
You will need: EBII ECU (just double-check the car it's coming out of has a separate g/box computer; some do, some don't. Ford.)
Smartlock module and plug.
Get the ECU plug too, you'll need to rip at least one pin out of it so you can connect Smartlock to the ECU.
Ignition barrel and key.
Consider door locks too. They are nonessential, but having the one key for all doors is nice.
Then, wire it all up, diagrams are on Fordmods.com as this is a fairly common conversion.
Personally, I'd stick an aftermarket one in - less work :) and also you can get some pretty cool ones these days which will page you!
Proud TQE Operator and President of SEFAG - the Society for the Elimination of Ford Automatic Gearboxes
Pacemaker headers + 2.5" exhaust
EF Fairmont interior
Lowered King springs
5 speed manual
Newest mod: EF Ghia wiring loom and dash