Hi Roger, I would have thought your engines spec (cam, head exhaust mods etc.) would have more bearing on your map than the small difference in capacity you are talking about. A 4.2 engine is actually 4275cc which is pretty near 4.3. I'm sure Nigel will tell you his maps are good to get an engine running and the fine tuning is to be carried out by you using the helpful tips provided on this forum.
Cheers
Steve - long time lurker, first post!