Did some googling and have not found anything on the front rail number. Car was built in March of 66, or at least when it was ordered, so I suppose April 66 could be accurate. There has been some cobbled up welding at one time in the past 40 years around the upper control arm, which I am fixing now. based on some damage to the original hood, looks like it had some front-end damage, and then tweaked on the frame rack (which likely explains the cracks behind the lower control arm mounts). The front frame & engine cradle were orange, which I assume was either "chevy orange" from a replaced clip, or someone went with a "tiger orange" theme at some point. The front body mounts felt like they had been in for 46 years as I twisted one off replacing them, so doubt it every had a new clip.
Have a Merry Christmas!