Everything I have read is, that you need 3/8" lines for the overdrives. Something about the added volume needed to help keep the OD unit cool. Every 500-518 lines I have seen are 3/8?, must be for some reason, if I'm wrong.
Either that, or it could be since they were being used in pickups and vans that chrysler anticipated that people were going to load them down and tow stuff with them. This is not normally so with a passenger car from that same timeframe.
If I had to guess , the 3/8" is to slow down fluid velocity so it stays in the trans cooler part of the radiator a little longer. I will be adding an auxilliary transmission cooler to this setup anyways.
