Lay a straight piece of metal accross the back of the wheel you have now and measure from the hub to the back of the metal and add an inch to that and that should put you back where you are now.
Another way you can do it is after you get the rearend installed lay a flat piece of steel accross the front of the drum and measure from there to the spring. If for instance you have 5" get a wheel with at least 3/4" clearance or in other words a 4-1/4 backspace wheel. Somewhat less backspace will work also.