@550 Phil I have a 400mm but a 420mm was the size that was in most cars of the era and your right the big wheel helps with worm and roller steering boxes and I like the look which should not be a problem.
I keep pondering a 420mm VDM/Nardi this one below in a spyder from Petrolicious is solid spokes but the normal VDM on the Classico wheel site has Nardi style spokes but with a 9 hole hub and the horn is either closed like below or exposed and then there is the rivet question of course.
Personal choice is everything as you know,
but 400mm = 15.75inches.
and 420mm = 16.54 inches.