I run my 275/40/17's on a 17x9 with 5" of backspace and have no rubbing issues now that I've upped the camber a bit (-1*). I would say that 5.25" backspace for the 18x9's would probably be pretty close to ideal, will let you go a little lower before you worry about fender interference and should still keep you clear of the frame.
With drop spindles I don't think you'll clear the tie rods even with 18's, so, again you'll probably be limited by the tie rod clearance. My 17x9's with 5" BS worked with my magnumforce drop spindles, and with the stock ones on there now. With the Wilwood's I think you're looking at an extra 1/4" of track width, so that'd put you at 5.25". I think on my setup my max backspace on the 9" rim would be 5.25", and that might even depend on the rims manufacturer. So, assuming the Wilwoods really do add 1/4", I'd say you're looking at 5.25-5.5" of backspace up front with the Wilwoods and drop spindles. With stock spindles you could run more backspace, the frame would be the limiting factor then.
In the back you'll need WAY more backspace than that. To put the mounting surface in the middle of the rim on a 12" wide rim, you need 6" of backspace. If you didn't shorten the axle, I'd agree with Travis, you're probably going to need at least 7" of backspace.