All the Osborn produced Mustang Assembly manuals are useful. I've worn out my 66 body and electrical (I had three 66's at one time) and bought second copies. Where they are especially useful is in the use of an old device called a "pencil" to note changes, additions and mistakes. Pictures help but you are never sure what a previous owner did, plus anything that you did as well, so take them anyway (thank "you know who" for the digital camera).
Jim
One more thing - If you want to get closer to original, you might consider adding the harness part number label, most of which are available (although their accuracy might be suspect), to the harness as it is not included.