I think you would get the best results from measuring the centers for border lengths, pinning them carefully to the length and width of the quilt. Pinning will help control stretch. Sew with the bias edges on the bottom next to the feed dogs. If you need to hand baste, do so. Some of the fullness in the bias HSTs will go away when you quilt it. I think you will make your situation more difficult if you use iron on interfacing because you will be stabilizing the bias rather than controlling it.