Very common fault with these engines unfortunately.
With 300k on it you most likely dont have the original engine.....without some sort of over haul having been done on it at some stage.
Diagnosing a liner slippage is very hard.
Do you have any signs of weaping (coolant) between heads and block? its very hard to get a good look, but try. If so you might just be lucky and its a head gasket.
I have heard of pretty good results with Irontight being used to sort this issue out.
Radiator; it is possible to pop the top and bottom tanks off as they are only clipped, its risky doing a re assembly, I replace my radiator about 2 years ago as I had damaged the expansion tank inlet on the top drivers side. I pulled my radiator apart just out of interest it was 95% good only 1 tube was blocked and another 50%, I have had my RR for 13yrs and it well worn but well serviced 345,000k now a bit rattly from the engine but she pulls strong uses no oil or coolant at all between changes.
Here is a very good company in the UK.
Read the bottom of the page on liners etc.
RPi Engineering - V8 Engines
Good luck