OZ SD1 and I think also SA used the 8.13 - 1 engines (or slightly higher). The LR engines have a higher water pump but the difference is in the timing cover not the pump (till later engines). On disc braked LRs the front disks/calipers may also be exchangable.
The main requirement is to get a V8 model LR/RR/Disco. Virtually any engine can be made to fit and the camshafts were the same up to near the end of 3.5L production but become tighter and tighter with each model after that. For a Low compression SD1 the last cam is probably the best for day to day city driving (4.0/4.6 circa turn of the century). Note they are fuel injected engines so you will need to find somewhere cool to put the computer (engine bay in SD1 gets very hot}. If yours isn't currently injected, the computer on injected cars is in the passenger footwell under a false floor/toeplate and the HP fuel pump is on the transmission tunnel down near the tank. You need plastic lines from late models as the steel lines vapor lock due to aforementioned heat.
Re gearbox choice The leyland P76 guys have that covered here:
Technical - Gearbox Index "Leyland P76" P76 used Borg warner boxes and there are aftermarket housings for T5 available (Dellow?) and that site names buick/olds/pontiac cars with T5 box and the same bellhousing mating face as the RV8