Thanks all for all the interesting advice on the best way to proceed. I think all things considered, it is best I stick to the V8 engine I have got as:
- it's going to be expensive swapping it for a smaller sized engine,
- for not much gain in terms of fuel efficiency, and
- I also like that it is a classic engine and swapping it for something else almost feels sacrilegious.
Thanks once again for the interesting discussion.
Rob