I'd like to see a larger thread into the main gallery, but it doesn't look safe (see Doc's articles).
3/8-18 NPT is up to .545" ID, which is pretty close to the actual passage ID, so it's not that bad.
However, rather than a small ID hose fitting as generally used (which restricts flow in the fitting), 3/8-18 to 1/2" ID hose fittings are available.
Don't use any actual pipe fittings to make 45 or 90° bends, the angle inside the fitting is very restrictive, use high quality Russell, Earl's, etc. stuff with formed steel tubes. If you enjoy fabrication, you can bend steel lines to work around the mount, follow the block etc. neatly, then solder the ends to accept a male NPT fitting and attach with a swivel, union etc. They should attach to the block with Adel clamps to protect against vibration fracture. Braided steel line is very attractive, but complete overkill due to the low pressure.
The filter base is typically a casting with fairly rough interior surfaces and sharp edges, casting flash, etc., which can be ported and all passages and intersects radiused.
The oil pump's pressure relief valve will still control the system pressure. A big line can't reduce it, because it goes back to the same main gallery. A small line will reduce the pressure after the filter.
