LS1 engine swap in Range Rover Classic
The LS1 engine fits in Range Rover Classic just fine. It is close to ideal in that it's an aluminum v8, quite compact and powerful. It is very reliable. The GM PCM (Powertrain Control Module, which is GM speak for Engine computer) can be easily tuned/reprogrammed, at least in the US there are plenty of places that offer this service. I would discourage going the carb route.
The LS1 (5.7) engines from Corvettes and F-body (Camaro, Firebird) cars are readily available, some are take-offs as people go to LS6, LS2, LS3 and LS7. You will need a wiring harness and a GM PCM. The problem is that even the least powerful version of LSx, namely being LS1, still makes more power that the LR drivetrain can handle. So don't bother with better flowing heads and other performance upgrades. Although, if you upgrade your entire drivetrain you could use the more powerful LSx engines.
Transmission - 4L80 is the stronger one, bolts to LS1 without any problems, and a company in Australia makes an adapter for the LR transfer case. It should be possible to fit 700R4 or 4L60E although I am not sure about the adapter for these. The ZF 4HP22, even when upgraded with stronger parts from 4HP24 (bell housing, torque converter, etc.) is still no match for LS1. Somebody somewhere probably makes an adapter for mating LS1 to ZF 4HP transmission, but I would not recommend going this route.
The Disco uses a different transfer case than Classic. Classic uses Borg-Warner and Disco uses LT230. The adapter for 4L80 was for LT230, so the Borg-Warner transfer case was replaced with LT230 at the time of the swap. IMHO the LR transfer case is iffy, it was never designed for this kind of power. It is likely the weakest point of the swap. I would be surprised if it held up in the long run, given almost double the horsepower and torque. Stronger transfer cases are available for 4L80 and 4L60E. You will need to fabricate custom driveshafts, which is no big deal for a competent 4x4 shop. The stock LR driveshafts and u-joints are too weak for LS1 anyway.
The LR axles are too weak, even the beefier versions found in later model LWB classics with 4.2 engine. You will need stronger axles, Mosers are the best but they are atrociously expensive. Stronger axle shafts are available for LR axles, but the diffs won't hold up for long with LS1.
The LR power steering pump produces lower pressure than LS1 power steering pump. This minor nuisance can be solved with different diameter pulley and changing the regulator valve.
Basically, if you want the truck to be reliable, by the time you are done modifying the only pieces left from the Land Rover are frame, suspension, body and interior.
Long story short, an LS1 swap is a lot of work, not worth it unless you actually enjoy doing the fabrication and engineering.
P.S. this is all second-hand knowledge, I am not the person who did the swap and I am not guaranteeing this information to be 100% accurate. Take it with a grain of salt.
P.P.S. Please don't misinterpret my words about weakness of LR drivetrain as knocking on the Rover. It was never designed for a 350+ horses, so it is unable to handle it. If you go with a modern V6, like ford Duratec you could probably retain most or the LR drivetrain, but the swap would be pointless as you would spend a lot to gain almost nothing.