OK, well, your query begs a few questions:
First, When you say swap from carbs to EFi, was your mechanic saying to swap the whole engine, or just the carburation system?
Next, you say that the 3.5 carb motor is the factory fit; I presume, being in a 90, which you dont tell the age of, is around an 85 or later...
Suggestion that "ZF Auto box doesn't behave that well" is a bit vague too.... in what way doesn't it 'behave'?
Could be that that is a seperate issue / problem that may or may not be effected by an engine swap or carburation revision.
The ZF-box was also used on both 3.5 and 3.9 variants by the factory, with the change points valved for the appropriate power curve, in original vehicle, most often a, heavier than a 90, Range Rover, which might account for changes being a bit more 'clunky'... if thats how its 'misbehaving', though old automatic transmission fluid, clogged filter or tired clutches can all add to box behaviour.
I presume as you said that the carb-engine was factory fit, but didn't similarly qualify the same for the box, that the auto-box is a retro-fitted conversion..... If so, do you know the oragins of the box? And, do you know anything of its age, mileage, service history etc?
Lastly, you mentioned that your mechanic was an LPG specialist..... and then asked defreyne, whether he was running on gas.... So, I'm guessing; were you asking your mechanic about fitting an LPG kit.... and was the suggestion to go to EFi in response to that?
BECOUSE:- If you were asking about an LPG conversion; first of all, yes, LPG can stand higher compression ratio's as it has a higher octane rating than unleaded; in fact its got a higher octane rating than 'Super-Unleaded 97; and you'll get more from the stuff if you have a higher compression ratio engine.
Hence clarification over your existing engine, becouse the EFi engines did have a higher compression ratio, though I believe its 9.35:1 the same as the 'last' of the carb engines, which you may already have.
But, if installing LPG, there are benefits to having an EFi engine in the fitting and setting up of the system, which are independent of the compression ratio.
On a carb V8, with twin carbs, you need two mixers for the LPG, one for each carb, and the physical 'plumbing' to get them to fit, and work with an air filter can be a bit awkward.
On an EFi, you have a plenum chamber, with a single air-inlet, to which a single, larger, mixer venturi can more easily be added, and still run the standard air-filter.
Then, on a carb model, when you switch to gas, you need to stop the carburettors supplying petrol, as the mixer starts supplying LPG, and with two float bowls full of the stuff, getting things to work well and give a 'clean' switch over, can be a bit more nadgery to do, than with fuel injection, where you can litterally just switch it off and go straight to gas.
So having EFi, could make it an easier, and possibly cheaper conversion, that might be a bit more user freindly.
And, to get EFi, may be as easy to swap the engine for an EFI one, rather than try swapping the carburation system over on your existing engine, as the cam-profile in the EFi engine is different, and the EFi system is mapped for that cam, and the presumed power curve, saving any burying into the engine to change that, or muggering around trying to change the fueling profile on the EFi control unit.
To which, conclusion, as your ZF probably was attached to an EFi engine originally, offers the suggestion, that if the box is a bit 'clunky', it could be that the change points aren't ideal for the carb-motors power curve, and so, having the engine that the box was valved for on the end 'might' see an improvement.....
Which is all supposition to suggest why your mechanic might have offered the suggestion he did, and that it isn't complete tosh from a wise guy to a mug..... there could be some pretty sound thinking in there... IF my guess work isn't too far off!
