4L80E Cooler line size 3/8" or 5/16"

[ZEKE68]

Have found that GM used 5/16" cooler lines on the 4L80E until 1998. They must have gone to 3/8" when they moved the return line to the middle of the transmission case. All the radiators I have found for my 67 to 72 truck have 1.2" - 20 threads for 5/16" cooler lines so there will be some restriction at the radiator even if I run 3/8" lines to it. Thinking about splitting at the radiator and adding an aftermarket cooler in parallel to keep the flow up. Since GM ran 5/16" lines on the 4L80E in the early years I assume that unless I work the transmission hard I would be ok even if I stayed with the 5/16" lines.