As Joey mentioned 5.7 chebbie install which has become a popular and cheap item over the years.
Your lucky to have the 4.6 vs 4.0, don't forget a 3.9 WAS in my 95 D1 5-speed. Same year D1's with auto were a total **** vs the 5 -speed. My vehicles must have manuals.
An aftermarket chip sold by RPI V8's across the pond if they have one for your engine also wakes them up, a big difference like going from a 3.9 to a 4.6 right off the bat.
Aftermarket cam, roller timing chain, mild head work, porting, cleanup and especially port matching including the intake and exhaust manifolds wakes them up big time. Don't forget an aftermarket exhaust like Borla's "Cat Back" as well installing Magnaflow high flow cats. Intake manifold extension project I machined now at 2,715 rpm max torque vs 3,100 stock. It pulls from 500 rpm's, pick any gear to 5,900 with full proper fueling by the RPI chip. RPI buys the chip off the inventor making a buck, recall Dave Smith or Brown the LR chip maker???
I haven't a clue what injectors you have, I bet Lucas dribbles. My case factory stock single hole Lucas dribbles of the 80's era crap LR used. I made up a balanced set (within 3/4% flows, 300 to 7,500 rpm injector pulse & duty cycle tester build a set of 4-hole Ford injectors which look identical to Bosch that again woke up the overall driving fun. Cold ram air intake another performance improvement.
I did this plus more to the 4.6 install back in 02 turning a slow **** 3.9 to a fun vehicle to drive. Best part it passes smog here in Kalifornia that is too clean on their **** smog machines.
Oh, years ago before the 5.7 chebbie installs became the hot setup, I was branded years before as an A-hole for trying to make a hot rod out of my "leave it alone they are slow" Land Rover. Last time I checked your 03 only had 217 hp, i'm at 255 hp on the engine dyno not enough to cause drive train damage unless driving it like you stole it. Torque off idle pulls way better than stock 4.6. I now have a engine that LR should of produced years ago. This a street use only D1 with stiffer cornering suspension the main goal.
My choice for engine swaps would be a 88 3.5 litre inline six gas BMW engine, with a little of work out to 3.7 litre with a Getrag 5 speed which BMW also use. An engine that produces 400+ reliable hp plus gobs of torque. My son picked up a 745 BMW engine (turbo 3.5) a gray market engine for his 325is BMW swap.
I bet a Rover engine swap would fail the visual smog inspection especially here in Kalifornia. That is why I believe in and kept the 4.6 engine.
.....~~=o&o>......