Got the reply from GA Spares about those blades, looks like they're a non-starter:

"it is only a slight step, its around 2-3mm."

So not viable at all for that type of tiny difference. So really it's either something like those Cox blades, which still will need the bolt hold enlarged and to but cut down in length. Or make some type of a spacer ring. Or perhaps just concede defeat with the H660 as a whole and move on from it to a smaller SP option for the sloping areas & in the hottest part of Summer.

Not sure of the best option at present. Bugger.