That is odd. The 18x9.5 come in 2 different offsets. By the numbers they both 'Fit'.
You must have the 40mm offset.
I can see the design of the Wheel has the spokes sweep back toward the edges to create that 3" lip.
Nice wheel....a case of try before you buy
A 3mm clearance is the accepted 'safe' measurement.
A 6mm stack of washers
...generally 5mm is the most you want to go, but if you measured 6mm and get 6 turns hand tight. By all that is written you are good to go.
It could be the wheel surface slightly thinner allowing the lug to tighten properly.
But use a proper spacer, it would be good to be able to try a 5mm first...a 6mm one would be a custom job.
and, sorry to ramble, a 5mm spacer would cover the hub center ring I believe. Find a hub centric spacer.
finally, you can buy extended reach lug nuts which would give a couple extra turns.
I think I got it all