I have found that the older fabrics from the 70's were poly/cotton! And yes, they do stretch! your best bet is to measure across the middle and then across the middle of the down side. Create a muslin lining for it and attach it to the muslin with your sewing machine and lots of safety pins to hold it for you. If you have a seam which is beginning to frey then top stitch it down. If you can hide your stitching in the ditch that would be best but if a block/triangle needs repairs do it with the muslin backing on. Then add your borders or whatever. I have had to "remake" alot of my own quilts from that era. That was the beginning when we realized we needed 100% Excellent quality cottons for quilts! Good luck!