Dave, 18" wheels were a factory fit on later-war WD16H sidecar outfits. These were principally supplied to the RAF (I have seen no evidence that the army took other than the odd couple).

When fitted with a 4.00 x 18 tyre, ride height was unaltered and wear rates improved considerably.

My feeling is that a solo 16H would feel over-tyred with 4.00 front and rear but you could probbaly get away with one on the rear and a 3.25 x 19 at the front (it wouldn't be pukka WD16H though if he's building a military bike).

If he runs 18" wheels with 3.25 tyres then ground clearance would be compromised and the geometry altered. Solo Nortons don't really seem to like 18" wheels in general.