old rotted frame rails out...new in!!

Something you can do to use the existing pieces.

Remove the rear shackle mount, marks where you want the upper mount to be. V out the bottom of the rail and invert the existing mount and weld it in place. Much easier than placing a tube through the rail.

Snubbers are garbage. JMO, haven't used one on any serious car in 30 years. Good springs and shocks are all you need.