Note that the OP is in England, and some choices might be really expensive there, though no budget was mentioned.
To my eye, it takes a pretty large wheel - 15x8 - to fill the wheelwells on 68-70 B-bodies and give them an appealing body/wheel relationship.
Plus, they were under-tired back then as pretty much all American cars were.
I could see a 17 or 18 but the OP wants to go with a more OEM look.
It would be cool to run something nobody else has, but a Charger in GB is pretty uncommon to start with.
Maybe have those Centerlines anodized red, or the Rallyes, or some big steelies, or a mag wheel that complements the body style...
To me, black wheels kind of disappear and lack interest.
I think some wheel sites might have a "configurator" to view a Gen2 Charger with different wheels.