I don't want to rain on your (or anyone else's) parade, but while this fantasizing is enjoyable, simply put, the P38 is the black sheep of the entire Land Rover catalogue and is born and dies as it is. No way to easily get around it.
I am "lucky" to live in a place where cutting and chopping is still a fairly affordable exercise, and I have seen all P38s projects "fail" one way sooner or later. Engine yes, I have seen later 3 liter BMW lumps no problem, but the hassles are in the transmission, and the project(s) are always showing their shortcomings.
First and foremost, we got the driveline on the wrong side, this means we cannot use anything from later cars - from the L322 onwards, the rear driveshaft and diff are where they are supposed to be, i.e. in the center. Ours is off-set to the left.
How would you match that? I had a friend attempting to overcome the issue by using a different fuel tank, move the entire drive to the center, and use a Patrol axle, but you need to chop the entire back lower body to fit it. He abandoned the idea.
I need to go back to my old correspondence, but I've exchanged a number of emails with David (Ashcroft) addressing the issue. There is no solution, plain and simple, except to stick with the ZF and hope it survives.
Engine wise, I had a 4.2V8 Audi lump from my donor S6 years ago, but there is not way to fit it. The Audi powerplant is low and wide, as it fits ahead of the axle, whereas the P38 has a fairly short and narrow engine bay, in spite of the car dimensions. A narrow powerplant like a BMW could fit, but measurements of the M60/M62 engine did not give me full confidence, as our axle has a fair amount of travel. A shop that specializes in protos and other crazy projects suggested go dry sump, but ... ultimately I thought to remove the RV8 from my donor P38 and use it as a test mule, but I do not think I will be able to get around it soon - if I take it to them, they are huge fans of the Merc M113, which is a fantastic powerplant and fairy reliable, and they could try to fit it, but the bill runs by the thousands ....
Also, I would stay away from the Vanos engines, they always (always) fail, no sense to put a troubled powerplant. And as the RV8 gives pain with its blocks, sleeves, and overheating, the M62 engine has its Nikasil/Alusil deterioration, the Vanos endless problems, among others.
Yes, it's got plenty horses to start with, but ....
And this without getting into the electronics hassles, ECM, BeCM, etc.
A friend is trying to rebuild his 4.6 trying to find components from other engines to make some Frankenstein with at least 4.8/4.9 liters, and hoping for more power. So far he's been at it for months, with no tangible result to be seen.
I got so depressed over the subject over the years that sometimes I want to get rid of it and just get a vehicle with a proper engine, but then every time I drive it I still like it and cannot feel the same emotion on another car....
Honestly, I think the best is to bite the bullet and make the RV8 reliable by having a verified block, fitting top hats, a performance cam of some kind, some proper work done on the heads, and perhaps have Mark Adams tune it.
I think this would provide sufficient confidence in the engine itself, perhaps aim at a 260/280hp (which I believe is the upper limit the Thor manifolds support) with a beefier midrange, and continue to use the rest of the vehicle as (poorly) designed without spending endless time trying to address all the other issues ....