The only bridge I've come across with 18mm spacing, apart from the higher end '3D' ones which offer adjustable spacing, is the Gotoh J510B0-5CK (
http://luthierssupplies.com.au/produ...roducts_id=447) which is a bit more pricey. The minimum saddle height from top of guitar body is 12mm looking at the specs diagram so may not be much lower than the stock bridge height anyway. Another thought I had to make the stock bridge saddles lower is to take the saddles off and grind a mm or so off the bottom of the saddles on the G/D strings. I haven't had a good look to see how much meat is between the bottom and the adjustment screw hole but it could be another option.