Measuring the rear from wheel mounting surface to wheel mounting surface is a much better measurement. Your rear should be about 62 inches wide wms to wms. Measure for yourself. 12 bolt 4wd rear ends work reasonably well. There are many many rearends you can use.
Some things yo will need to consider will be tire size, gear ratio, and transmission. 80 mph in a 50 is doable, but scary with the stock brakes and suspension. if you plan to go that fast you will need to upgrade your brakes and tighten up the steering.
