Post in the P38 section. 18" wheels are homologated for the P38 but you need to make sure they have the correct load rating, the P38 weighs 2.25 tonnes and you need the correct rim witdth, tyre profile etc to give the correct rolling radius or the speedo will be incorrect. Even if the stud holes line up, you may need new nuts and spigot rings.
The P38 handles better on 16"