Front Rubbing - Tires, Suspension or Wheels?
The tire on there now is a 295/50/15 and there is no distance between it and what it would hit. It already rubs on the tub and the fender lip (more so on the tub).
Go measure that wheel about three more times... if you are barely rubbing on both sides with a 295 and want to run a 275/40-18 it fits perfectly on a 9.5" wheel. What you want is the same offset. It sounds like your wheel is centered. Measure the outside of the wheel from edge to edge, a 10" wheel will measure about 11" outside edge to outside edge. If your 295 is rubbing a 275 will give you almost 1/2" more room on each side if it is centered.
Now, how to calculate offset... 11" divided by 2 is 5.5". If your wheel measures from the mounting face that sits against the brake drum to the back edge of the wheel 5.5" that would be 0" offset. Anything less is negative offset, anything more is positive offset. Use a good straight edge laying on the outside of the wheel lip when measuring backspace, it is easy to measure this wrong. I use my carpenter square. If you are measuring 5" that is -0.5" or -12.7mm offset. You want the same offset on your 9.5" wheel. Since a 9.5" wheel measures close to 10.5" that means 0" offset would be 5.25" backspacing. -0.5" offset would measure 4.75" backspacing. Another way to think of it is that your wheel is 1/2" narrower than the one it is replacing and you want to take 1/4" off both sides to keep is centered. That is 5" - 1/4" which gets us back to 4.75".
Make sure you get them to check the backspacing on whatever wheel you order. I was looking at Summit's website and the descriptions were all jacked up; the size, backspace and offset numbers they had listed did not add up. One of the numbers is wrong on several of their descriptions. Surprisingly few people can calculate offset correctly, so I usually stick to backspacing when I discuss wheels. That includes people that work with tires and wheels.
If they do not have the 9.5" with the backspace you need a 275 will work fine on a 9" wheel. Just take 1/2" off the backspace of your 10" wheel, so, assuming 5", that would put you at 4.5" backspacing on the 9.