Not sure which lines you are working on but my stock Th400 has steel hard lines attached to the trans and then tie into the cooler with flexible rubber lines. I could not find a replacement for on that leaked so I just put the fittings into a larger diameter piece of rubber transmission line and secured them with hose clamps. It has worked for the last 2 years. Not sure if this answers your question but I tried.
Gates has some hose. It is called lock-on hydraulic hose, rated for 300 PSI. The neat thing about it is you don't use hose clamps, just a special hose end that you push in. the only way that end will come off is if you cut it off. My recomendation is steel brake line, just like most of OEM does.