I am looking at my rust puzzle (780 with B230FT). The lines from the AW71 to the
rad are looking pretty rusty. Same for the lines going from the rad the to seperate
oil cooler. I have to change the rad, so I am looking at everything around the rad
which might need renewal.

Question, was the extra cooling really needed? What kind of pressures are running
in these lines? Can they be replaced with flex lines or DIY piping and bending?

The lines from the rad to the extra oil cooler are disco. The lines from the transmission
to the rad are still available but pricey.

Automatic transmissions can never have enough cooling. You can probably replace the hard lines with hoses, but there are specific hoses for this application. I'm not sure what kind of pressures are in a transmission cooler line - I don't think it's much - but if you tell the parts guy what you're looking for I'm sure he'll get you what you need. It'll be the fittings you need to concern yourself with more, though.