My date and I sleep under a standard sized Nunatak dual arc alpinist, and after comparing its width dimension to your JRB, I might suggest adding a down filled “wing” to each side of the torso section of your quilt. At shoulder, the arc is appx. 19 inches wider than the JRB, and if side sleeping, or changing sleeping position, I can imagine the 61” width as drafty.
Our quilt also has an insulated tongue that Neil wrote of, and having used a double quilt without, I would never have guessed the comfort that it adds to double quilt sleeping. It is an excellent suggestion.
The foot box girth of the Nunatak is narrower than our previous quilt, and although I think it restrictive, especially at the shin area, it is much warmer. If the JRB folds in half in the foot area, its foot box dimension would be comparable to our old quilt. (A converted bag).
Our Nunatak also has three girth adjusting ties, and although we never shorten them, each of us uses our respective ends to draw the quilt closer and warmer. Lying on the ties, with the excess between us, seems to keep the adjustments intact.
Finally, when temperatures drop into the thirties and below, my date adds her Reed rain pants and a wind shirt to minimize skin-chilling drafts while sleeping. She is much more tolerant of any casual bellows draft since.
After enjoying the flexible pleasures of bottomless dual quilt sleeping, we would take a few steps before adding a bottom sheet. In any event, good luck!