Hello again!
I think I am going to use the M&D kit, number 204. Land Rover conversions - M&D Engineering
It includes the adapter plate (or bellhousing if needed), the spigot bearing bush that Markyboy cleverly pointed, and the engine supports, it's a straight forward do it yourself kit.
The good news is that the 300 Tdi clutch disc fits into the 4jb1t flywheel (I just need to check the thickness of it, if it is thicker than the 4jb1t's, I'd just need to build and adapter (we call it "a flanger", don´t know if the word has the same use over there) for the pressure plate, if thinner, the opposite, machine the base a little so the pressure plate goes "deeper", so it gets exactly the exact pressure needed. With this configuration I wouldn't need to find a workaround to modify the power takeoff of the gearbox or build a custom clutch disc (my biggest concern so far, is gone! ) Do you know if the Defender 130 clutch disc differs from the 110 one? I'm planning to use the heaviest duty possible, for endurance sake.
About the specs of the 4jb1t, they certainly are not V8 like, but this engine is tuned for heavy work and long lasting durability (and certainly is fair easier and cheaper to maintain and way more reliable than Land Rover engines, please don´t get me wrong, I love the Land Rover heritage, but sometimes it just hit the pocket too hard to be that faithful). The tuning can be easily improved with a few small mods, like a good intercooler unit, more turbo boosting and a little injection pump adjustment. I've seen a Range Rover Classic with this engine, and it performs better than a 300 Tdi unit, even when the Rangie is a heavier duck.
Thanks a lot for your help, I'll keep you posted!
Cheers!