Custom 1/2" spacer w/ extended studs...advisable?

I am running a 1/4" spacer now with extended lug nuts. I just had BFG 285/65r18 tires installed on my rims (18x9, +18bs) but haven't had a chance to take off my winter rims/tires yet so I don't know if I'll have enough clearance on the UCAs etc. I don't think it will be a major problem anyway, but was debating on getting a friend of mine to make me a set of 1/2" spacers and combine that with some extended studs if necessary. Then I wondered if the bolts would be strong enough to allow the wheel to sit 1/2" off the hub given the spacer is just placed there and not bolted on like a 1.5" spacer. Anyone have any thoughts?

 

Whats the backspace on the wheels

 

Sorry, that should have read 5.71" bs and +18mm offset.