[rotorwrench]

That may not be a bad idea. I've done resizing of NPT fittings before and I've also installed heli-coils in some aircraft applications. When going larger a person sometimes doesn't realize how much larger they are until you have one in hand. I drill to the minimum thread valley sizing at the start of the taper. I then use a tapered reamer to open it up for even use of an NPT tap. A lot of care has to be taken not to enlarge it too much. If you ever do one in cast iron, you will find out how fun that can be. Aluminum is a lot easier but regardless of material, they can be tricky. I use mostly AN fittings. They are available in anodized aluminum, cad plated steel, and stainless steel. The price will rise in that same order. Many times brass can be sourced and there is nothing wrong with using brass if it will do what you want it to.

There is a lot of stress on a tapered type fitting at installation and removal so don't drill them any farther than you have to. A person can also source straight thread type fittings and use a spot facer to finish the boss for a small cross section O-ring lock-o-seal and nut to seal it up. These are custom applications in the aircraft industry but they are out there.

Flatheads didn't really need a lot of oil pressure to function. The old universal carriers didn't go real fast either. They were more like a fast tractor than a jeep. They could likely set there and idle for quite a while unless the air temp was too hot. 1/4" NPT fittings are usually used with 3/8" size tubing in aviation so it would only act as an orifice to slow the flow a bit.