Sustraley is it exactly 42x42 or is it 42 1/2"x 42 1/2". That 1/2" is for the seam allowances, if your's is 42x42 you'll need to make it 42 1/2" square. Just add a small strip around the outside and it will look like piping...should be fine, then the chain will fit perfectly. Your blocks will finish at 6" so 6 divided into 42" equals 7 on each of 2 sides and 9 on the other 2. At each end you will have your 1/4" seam allowance for the next round. Your quilt will be 56 1/2" x 56 1/2" when you're done with this next chain border. Hope this makes sense to you, if questions just ask and I'll try to give you an answer.
Emma if I do it won't be until February, with Christmas and all coming up and people finishing this one, hubby and I are going on a cruise Mid January and we won't be home until February 1st.