Dave
there is a differance in TUBE (od) and PIPE (id) sizes

The sizes you refer to are NPT pipe threads and are standard sizes for AN or compression fittings to tube sizes.

A 1/2 od tube with .035 wall thickness is .430 id or just under 7/16. A 3/8 od tube with .035 wall is just .305 id which I feel is small for most any performance engine

Jerry