I have a 68 Firebird coupe project I picked up a couple of years back that was someone else's project and had some pretty crappy floor pan replacement and other things done just to try and hold the underside together. I am a DIY and took on a big project here in need of some flooring and rail section replacing(learning as I go...). I have found a great deal of helpful info online with measurements, how-to, etc etc.

I am currently in the process of replacing a foot and half or so section of each rear frame rail. Left side was pretty badly rusted, the right side had been hastily replaced with a section of channel steel and shackle mount hole was off at least an inch or so! Anyway, I am not putting together a show car and am just wanting to make the structure solid again and have a fun ride. I hung some plumb bobs(sewing thread tied to new washers) to take some measurements last night with the new rail sections resting in place. I was pleased to find the front rail side to side measurement was right on at 33 1/2" from center of master gauge hole to center of master gauge hole. Front to back on each side from center of master gauge hole to center of spring bracket mount hole came in at 55" on both sides, and I thought that was pretty close as well considering maybe some slight centering error. However, cross measurements came up with 66 1/2" one side, and about 66 7/8" other way. This was puzzling to me since the other measurements were right on/same.

I looked at the top of the rails and took measurements from the tailpan/mount flanges to the center of the shackle mount hole. The left rail section had already been bothering me since it did not quite fit flat with the angle of the tailpan and was needing the mount flanges rebent/massaged to make them weld on flush. I measured from the back end of both rails to center of shackle mnt holes and discovered that left rail was a little less than 4 1/4" to center, while the right rail was 4 1/2" on center. I bought these at different times and from different companies. I think this left rail is not correct and am wondering if I could get some feedback on this measurement, and any input on what I should do moving forward. I found more than I bargained for once I started replacing the floor, and if I had to do it all over again would've probably went with a full floor pan and maybe complete rails instead of sectioning. But, just doing what I can on my own in my own garage one area at a time!!!

Have a look at my old thread: https://firstgenfirebird.org/forums...ll-and-align-68-subframe.html#Post313630

Thank you very much, any and all info is greatly appreciated! Although I am not putting together a show car, I am not looking to just cobble this together in a "good enough" attitude or approach.