Before you start to make a rear rim conversion - be sure to check the rim width recommendations for the size of tire you are planning. A lot of modern tires are being built to fit wider rims than were used on the older bikes. The rear tire can be up to about a 130/70-18. Beyond that width and height you are going to have problems with the swing arm being too close to the front of the tire. You might be able to use a 17" diameter rim - although you might need to have special spokes made. I know Buchanan makes spokes for the 18" rim.
If you put too wide of a tire on a narrow rim - you pull in the beads and make the profile of the tire too round, and it rolls the tread over onto the sides where you can never use it.
http://www.shinkotireusa.com/tire/705-series