Go to Unofficial C27 website.....
....at http://www.geocities.com/catalina+27/ There is lots of cool stuff at this site. You can really learn alot about our great boat.... now... go to "C27 miscellaneous", then go to "rig specifications"... the forestay length for C27 standard rig is listed as 34'11.5". However, most riggers, including Catalina factory, suggest a physical measurement if you are replacing the item. Or better yet, remove the forestay, take it to a rigging shop and have them duplicate the piece. Why? Because if it doesn't fit you don't get your money back! Have a ball.