I can't see that being a problem at 12mm considering the size of the wheels/tyres/hubs etc. - but then fizzy changed it to 28 - 30mm !!
I know - didn't measure it right before - included the tyre

Having said that plenty of cars and Landies are fitted with 30mm spacers without problem. Ok they don't have 3500kg on 1 axle but they dont use 22mm x 6 wheel studs either.
I suppose I could use 2x 15mm spacers - same difference and easier to laser cut I assume?