I once picked up a really cheap Breitling watch from Thailand.....
He promised me it was real....
I dont think you will get much sympathy here. Most are P38/RR enthusiasts who will keep them original to maintain market value and because they brought a P38 because its a P38.
The amount of time spent on butchering a P38 to make it into a sport makes no sense. You could quite literally buy an early 4.4 sport for the price you're going to pay in the end. So why didnt you? The engine will be far more reliable as will the rest of the car than a P38.
Instead you're wanting to drive around in a statement that says, "look i brought an old range rover because I can't afford a proper sport -but i did my best to make it look like a sport!"
If you're still hell bent on doing this then do google some conversions and see how bad it really looks when nothing matches up. Then google some up coming MOT regs which means some of the body parts fitted may mean the vehicle will no longer pass an MOT.
Then think of the resale value, if you, for aguments sake, brought it for 4K, spent 4K on it, you will sell it for 1K if your lucky (Heavy petrol engine, bodywork and framework hacked to bits, destroyed a potential classic). Even with my maths that's a loss of 7K?
Dont know about you but I haven't got that money to throw away....