If you need a four link rear you are also going to need a full ladder frame to replace the front half as well. X frames were designed to flex like a rubber band with the car body and suspension literally mounted in rubber to reduce road noise and any minor bumps. The frame flexes to account for added suspension travel. Great for a rock crawler but not of much value in any form of racing.
I used to talk to Scott Weney (the W of S&W Race cars) before his passing, and he was always most helpful in designing partial frames for my Nova race cars. I have also bough a back half kit from Chris Alston Chassis Works. Great quality as well.