You can buy a quality 8-foot single line delta for about $30 or less. Look on eBay. You'll find hundreds of them. The dual line you have is not really intended for single line, and although not impossible, the conversion isn't worth the headache and trial and error involved. Of course, if you want to give a try, go for it -- and let us know what you did and how it flies. Most of us who have done conversions have gone the other direction; from single to dual or from dual to quad.
Good Luck,