Clean the holes in the forklegs, IE. no rust deposits etc. Then check the spindle for damage, I found some burrs on the shoulder behind the nut thread on a 350. Don't hammer it in !! if it wont go in easily there's something stopping it.
Is the tubular spacer between the two bearings properly centred. Try the spindle fit without the wheel in place, that will give you some idea of where the problem is.
