You should be able to use the same shaft unless you have the wrong TH400 in length. Spicer makes a cross over Ujoint that is small one way and large the other way. You will need the TH400 slip yoke too and probably that same U joint issue unless you find the big shaft but then the back joint will be too large for the 10 bolt yoke
The TH350 and the PowerGlide are the same size. It was introduced in 1969 to replace the PowerGlide. It uses the same output yoke and drive shaft as the PowerGlide, and mounts in the same location as the PowerGlide.
The TH400 is a physically larger transmission (longer, taller, and bigger around, but it still fits in your car's trans tunnel without hitting anything if it was a factory option), and because it is longer the transmission mounting point is pushed further back in the car (1965-'73 cars all had dual holes punched to mount either a TH350 or the bigger TH400).
The TH400 has a larger output shaft yoke (more splines) and needs a shorter drive shaft.