'86 Faring on a 2005 Bike: Can this be easily done?
Yes it can. The fairings are all interchangable. The only real difference in the 86 (and maybe 87's) is there is a removable duct in the middle of the mid-fairings, where in subsequent years, mid-fairings were one piece units.
None of this comes into play when fitting them to any C10.
Can you retain the dash/instruments etc of the later machine and use the upper skin of the older?
Yes you can. If you have broken inner fairing parts, (gauge surround and the pieces containing the fairing pockets) '93 and older will NOT interchange or fit with either the newer ('94 and newer) gauges or the newer fairing stay. Inner fairing parts would have to be replaced with '94 and newer parts.