Yup- A totally new frame that's not as tall as the original, with lower body mount locations is how a SFBD is done. Often times the only original part of the frame that's retained is from the firewall forward because it has the control arm mounts and engine cradle and it's all above the bottom of the body. There's lots of careful measuring and fabrication involved, but you don't lose interior space like you do with a traditional BD where the body is channeled to lower over the rails. It's definitely had some hours poured into it.