Cory,
Even with an open trailer, you really need the 6.5 width. I had a single axle 6.5 x 12' (yes 12 footer) that worked well..just be sure that the fender height will clear the speedster door bottom ...you can aslo cut and remount the left trailer fender so that it is removable ....These trailers are fairly cheap on ebay at $999 or less Ramps ..just use two 6' pieces of 14" wide "LVL" Lamimated Beams that look like 1 3/4" thick plywood....available at most any lumber yard ..they may even have cut offs you can get cheap.
Alan
This one is a 16 footer but you get the idea,just cut off the side angle stock
http://cgi.ebay.com/ebaymotors/NEW-6-5x16-CARGO-UTILITY-TRAILER-LANDSCAPE-CARHAULER_W0QQitemZ4643346035QQcategoryZ80770QQrdZ1QQcmdZViewItem