The Original Terminal Box (72325-48) was bolted to the side of the frame with a cover (72335-48) over it. This was bolted to the frame just above the pivot point of the rear swing arm. Not very secure.
You may think about using the Terminal Block (72311-70) that Harley started to use with the 1970 model year in addition to you using the original box. It was mounted on an angular bracket (72310-70) inside the headlight shell to the top triple tree. This gave some security to the wiring. I know this is not "Stock" and "Original" but this does give a better wiring set up than the connections used on the "Box" Harley used from 1948 to 1969.
And Please get rid of the Clymer and get the Harley Service Manual, 1959-1969 Electra Glide Models (99482-69) and the Part Catalog for FL Models, 1941 to 1982 (99456-82). It will give a much better chance you will get it right the first time. You can still buy them new from Harley. Or look around the web, like eBay.