Rear wheel WM2 vs WM3

WM2 inside 1.85" (47mm) outside 2.72" (69mm)

WM3 inside 2.15" (55mm) outside 3.080" (78.2mm)

There is not much space to the left of the tyre (3.25 x 18) between it and the mudguard, so stick with WM2, and use an old fashioned 'square looking' tyre. A modern one may rub on the mudguard.

Czech company Mitas make exactly the right tyre:

https://www.mytyres.co.uk/rshop/tyre/Mitas/MC7/3-25-18-TL-52P-Rear-wheel--Front-wheel/R-269204