I agree with Kassaundra. If the fabric is 108" and you it is non-directional, you can use it for the 96" direction so that leaves 8" extra there. To add the same to the 84" makes 102". Divide that by 36 and you can come out with a little less that 3 yards. 3 yards should give you what you need for the back but do you need anything for binding? and since this is an old quilt, I would want to wash that yardage to be sure of shrinkage or possible bleeding before it is finished. Also, if it is considered vintage, I would carry through and quilt it in the method that you are restoring. If it was hand quilted, as most were in the past, I'd do that again.