th400 Cooler lines
http://www.summitracing.com/parts/ati-925137/overview/
Run a -6 to a plate cooler. It'll flow plenty. Monitor your temp gauge and up-size the cooler if necessary.
If you calculate the actual area of the tubing the 3/8" is a much larger capacity and flow.
In either case the restriction will typically be in the fitting themselves, not the size of the tubing/hose.
Don't overthink it.




