if you take the back tire off you can hang a plumb bob off the inner wheel well and one off the outer fender.as long as they hang below the axle.if you measure what you have between the two strings that is the widest you can go.unless you dont mind the tire sticking out a littl bit.leave yourself about half an inch or so on the inside so you dont have to worry aboutrubbing and measure from the inside string to the face where the rim will touch when it is bolted on.that will give you your backspace.keep in mind how much the tire sticks out past the rim.if you come up with a crazy b.s. dont sweat it ,you can add wheel spacers.its cheaper than custom wheels.if you lat your exsisting wheel with the outside down and measure from the rim to the face of the rim(where it touches when bolted on)thats the backspace you have now.there is a tool the will measure all of it for you but they are expensive unless you know someone who has one. | 
how much backspacing? 
