Bungle and Ethel,
You have both replied in relation to the spacers shown
This is not the question
The question is, with the wheel moving out, by whatever means, it is no longer tight around the centre hub
Is this safe, being held in place by the four studs?
I think you got a fair answer - but I wouldn't rely on the hub centering the wheel - as stated, bolts work by being taken to slightly under their elastic limit to clamp, however the wheel is held clamped again the brake drum in this case by friction and the centering provided by the studs themselves and the taper on the interface between nut and wheel.
Bearing in mind that you are hauling slightly short of a tonne on singles, I'd tend to get decent spacers made up out of either steel or ally, ensuring that there are countersinks for the nuts to sit in (your wheels look a bit the worse for wear on the cones) - if the hubs are standard, then at least buy decent quality extended studs.
I personally would look to finding wheels that gave you what you wanted WRT offset without using spacers. There would be a lot to be said to using sleeve and washer type wheels if your going to stick with spacers.