I am not sure what you are saying there. If you can fit a 4.00 x 18 on your bike, with half an inch clearance, why do you want to run a front tire in there, which is, what, 3.25? Why not just run the 4.00 like Norton designed it?
I have ended up using a conical hub 19" rear wheel I had on hand and fitted it with a 4.10 x 19 Dunlop TT100 and that fits with about 1/8" to 3/16" clearance each side.
This is what an Atlas framed bike looks like with a front tire on the back. That is a 3.60 x 18 I had on there just for the purposes of wheeling it around the shed. Just looks anemic to me.
[MEDIA=imgur]pcsY17N[/MEDIA]
The Atlas in the background has a 4.10 x 18 TT100 on there and even that looks a bit anemic to me. It is 4.100 wide as measured, but of course it is a low profile tire so does not fill up the space between the tire and the guard. And you lose a little cornering clearance as the bike sits that half inch or so lower.
Not a very good angle, but here is the 4.10 x 19 TT100 by comparison. It fills up the space better because it is the same outside diameter, more or less, as a 4.00 x 18.
[MEDIA=imgur]yPCynHF[/MEDIA]
