I just assumed there would be more backspacing. Also I'm unsure of what width to get since I don't know the width of the current wheels and can't measure them while they are mounted.
The wheel width should be stamped or cast somewhere on the backside. Also, the wheel width can be measured. Lip-to-lip width is almost always exactly 1" greater than the "wheel width". You might have to lay something like a foot long 1x3 board across each lip, measure the width between the boards, and then subtract the width of the 2 boards, and then subtract 1".