Well I bought new rims too from the us.
I sold the Cobra R's I had on the car. I wanted a wider tire in the rear and like the look of the deep dish. I could get a 17X10.5 Cobra r but not in deep dish (you can get r's in deep dish but they have rivets and wouldn't match the fronts).

I ordered some chrome FR500 rims from AM.
The 2 17X9's were fine but one of the 10.5's had 4 bubbles in the chrome. You could also actually see the grooves in the chrome too where the rim was machined when it was made. The other 10.5 was flawless.

Anyway, I emailed them about the problem and they sent out another rim the same day. Hopefully they checked this one before they sent it though.

Would a 255/40/18 fit on a 05-08 Mustang GT with a stock optional tire size of 235/35/18 ?

I'm not sure of the size you listed.

As far as I know the 05/08 GT has a stock size of 235/55/17 with an optional size of 235/50/18.

I think you would have problems running the 255/40/18 as the diameter will be off by over 4%.

This link might help.

www.1010tires.com/TireSizeCalculator.asp

What did the tires cost ya, if you dont mind me asking????

PM is good or spill your guts here......lol

They were $98 each and $200 for taxes/shipping/duty.
Shipping probably would be a fair bit cheaper to send to Ontario for you though.