Just to correct a few things early P5/P6 V8 could have either stromberg CD175, zenith 361V, or SU.
Most rover SD1 were carb not injection inc the V8S, VP and Vitesse were FI, though a shit system such as lucas L4CU mostly flap and hot wire some better than others.
If it is a P5 engine the first thing you need to do is replace the rope seal rear crank bearing fort a newer sort, there are several design differences down the years but all basically the same, mostly down to age.
Millions of range rovers were carb as well.
When BMW owned rover they improved the V8 with close fitting caps on the 3950cc engine, these had the facility for cross bolting but this was not used on the 3950 just the 4.6, though 3950 blocks could be converted to fully cross bolted.
The ignition system was not at all designed to operate at 9V leaving more in reserve for starting, it was merely down to the fact that voltage under cranking could drop to 9v, using a 12v coil with 9v isn't going to be very successful!!
If it's carb it is a lot easier, but fi or efi is till not hard.
Personally I would use the LT77 or R380 box as originally fitted on the rover V8, if you can the leyland sherpa V8's as supplied to the plod and ambulances etc use a slightly better version of both boxes, they are quite a slow changing box, but even a mildly tuned 3950 is going to around 250+ lbft at less than 2000rpm, it will shred a type 9 but won't bother a LT7 or R380.
As for the diff the bog std diff can cope, in fact a 3.14:1 has been on the V8 xr4i sat outside my window now for 6 years or so, and it does get used hard, gearing would be the biggest bug bear, way too under geared with 3.62:1, we also tried my 3.38:1 but this too was way under geared, makes a hell of a nice easy to drive fast car.
The heads are a bit of a disaster, in flow terms, and the most common cr is only 9.35:1, so a 3950 block with 3.5 heads works well.
I wired my dads one in in less than 2 hours, and it started first turn of the key, we used the std return to the tank for the feed to the carbs through a pressure regulator, we use a facet red top pump, engine mounts are easy to make, I used land rover cotton reel engine mounts simply mounted onto the std cross member under engine and made plates type mounts to go on engine.
The V8 is a long engine so you may/can have clearance problems at front, so use the shortest pulley set up you can on the engine.
The std prop can twist like a chew and snap clean in two, trust me I did it, so need a decent one making, the std, non CJ13 joints on the prop will break apart.
The most annoying bit though is the manifolds, most likely will need to dent it in around the steering shaft area, gearbox support can easily be made from the std sierra one and the std rover one with cut and shut.
Tacho can be altered for around £55 by speedy cables, can't remember what cable we used think it was either the std ford or std rover one fitted both ends ok, on a Lt77 box anyway, the change is slow so it is actually quicker 0-60 starting in 2nd gear and slipping a bit, pulling away normally in 2nd even with a 3.14:1 diff doesn't bother it at all.
The 3.14:1 diff was from a sierra mk1 diesel 2.3 estate.
tabetha