as gordon has mentioned, all that stuff is available, and im sure someone on here has replaced it in part or all, and has it documented. or at least has the specs. from a stock rear section for referance.
and as far as referancing the rails, its all about front to back, and keeping it square, they both end equally at the tail panel.(tough for me to explain on a comp)
custom frames are not a restorers dream, but they arent that bad, if executed properly.
you alone can decide wether you'd like to build a driver, or restore the car, that in itself is a world of differance.
best of luck in whatever you decide.