Level of difficulty, 8 out of 10 for a me, possibly 5 out of 10 for a technician.
In my opinion, its simply not worth loosing 20 kw, and doing the other mods at all.
The EA electonics are really well sorted out, and the plauge of rough idle issues, and leaking plates on the TBI unit are really easy for a technician to fix. As far as EFI units go, the EA is just childishly simple, and will do you no harm at all. The XG uesd the engine, and the Mutiple Point EFI engine is very strong at about 139 kW, and its a 15.8 second road burner with a 3-satge auto, the optional 3.23:1 diff, and a set of Genie extractors. Mick Webb used to have his autos quicker than stock 5-speed Falcon EA 3.9 S-packs.
Okay, enough preaching.
What you are doing is adding another carb and locking of the throttle body EFI. Then doing a 'fontal labotomy' on the non critical electronics. No different to what Taxi Gas conversions used to do in 88 to 91 on the EA and EAII's, but there are some issues.
Basically, just use the Weber ADM 34 carb from 1986 to 1993 (XF sedans and wagons to 1987, utes from 1986 to 1993). Use the later XG Ute 3-way cat and exhast.
A few screwy things with early cammer Falcoons. If you are using the T5 5-speed, then use the XG Ute spec item, clutch and cable. If you are using the 4-speed BTR LE 65/85, then you need to fit an electronic Throttle Position sensor (TPS), and thats not easy to calibrate because the Weber carb is staged, and the TPS has to be calibrated to suit by a technician, or the shift computer will pull a hissy fit on you.
If it's the last of the BW 3-speeds, you should be sweet as. I think it may not have had a TPS No major issues with this Noahs Ark throwback, as its strong, and needs just the XF kickdown lever. I frankly can't remeber if the last XF's had TPS sensors for the EST. I know they had manifold absolute sensors.
In my opinion, the process is
A) The EA Throttle body injection (TBI) gets removed, but you stick with the EA intake manifold.
B) Rather than mess about with the the clashing bolt patterns, just get a taller 2300 Holley 2-bbl 25 mm alloy spacer available from Redline, and have a machinst mount it to the manifold with the TBI bolts. Then fix a 5 mm alloy plate (free-machining or tooling plate) to the Holley baseplate using the wide based 5 and 5.125" bolt spacings. Then the Weber ADM'S 46 by 93.5 mm bolts go on there with a set of countersunk bolts. So its sort of like a three bolt pattern, two layer sandwhich to get it to fit. Not hard, just needs a little nauce.
C) You add the Weber carb and mate it to the TBI or XF air cleaner. The throttle linkage and intake need to follow XF practice, and the Throttle positon
D) The Weber ADM will work fine. The EST and basic engine systems from the 4.1 will swap over to the EA engine if you really must, but is better to use the stock ignition system from the EA. LPG fitters tend to use the stock EFI/TBI systems, and add a propane carb on these EA engines. Been doing it for years. Better to use the TBI bits, excpet for the carb. If you do that, and set the O2 sensor on open loop, then you should be able to do it.
E) Get a lower pressure electric fuel pump. It will need to be modified to deliver 5 psi to the carb, and then the stock return line and bleed back oriface in the carb will reduce the peak delivery pressure to 3 psi. The early RWD electric Holden Gemini pump should work as a good alternative.
*The whole deal with TBI is that it gives poorer atmomisation that a carb, but is easier to calibrate and control for the emissions. When Ford introduced the EEC computer module in US cars in 1979, it was the end of carbs becasue it allowed easier emission calibration.
One guy on the US forum, who did dynamometer development work on the Central Fuel Injection (CFI, or TBI) 5.0's and 4-BBL HO 5.0's said the CFI had a poor atomisation, while the 4-bbl and especially the multi point EFI were much better. Carby's are therefore not a totally backward step, but you'll loose 20 kw becasue the carb is jetted for the 4.1, not the harder reving 3.9. Stock 4.1's were 97.5 to 103 kW, while the 3.9 TBI was 120 kW
He actually slapped me! 
