Difficult choice, Tony.
I'm a bit biased (or do I mean lazy.....) as I don't have the petrol engine fine tuning skills (I mean patience ) of others, so take my take on it with a pinch of salt, but regardless of how you weight the benefit of diesel economy, there are other factors which have an impact. For example it seems to be increasingly difficult to find spares for many of these big wartime petrol engines. Fair to say I think that economy on its own, whilst definitely real, depends on amount of use the truck will get.
Diesel wins on: complete engine availability / reliability */ spares availability / rebuild (engine machining) capability / economy
* especially after a long period of layup when petrol requires a lot of attention - spark, stale fuel
Petrol wins on: originality (including the sound!) / ................there must be more but nothing springs to mind!
How about the following rationale?
If the truck is a pristine restoration or one which has good potential to be made into same, an original petrol might offer best resale potential.
If it is a good useable truck which will probably never be a 'minter' (being realistic a lot of trucks are in this category) then a diesel might give a better resale value or appeal to a bigger potential market?
On the other hand, if the truck is going to need a lot of work by whoever buys it, then whoever takes it on would probably have the skills to do the conversion should they so choose, and if it has a recently fitted good RXC this should have a good resale value to those of the original persuasion.