Nick - Don't shoot me brother, but I'm going to have to back pedal on my above post.
First - The diagram I posted does show a later plumbing connection setup from the the brake line to the master cylinder with what is known as a "banjo bolt" (4.661) and brass connector block (4.686). I dug out a combined 1942-47 Car/1942-46 Chevy Truck Shop manual and that setup is not evident on the master cylinder pictured.
Second - My indicating that your truck would use a "pressure style" brake light switch (added notes in the screenshot drawing) was incorrect as the shop manual indicates that a firewall mounted pedal operated switch was used in your truck. Eliminating the pressure operated switch removes the need for the banjo bolt and brass connector block, which were both used in later trucks (but not associated with a pressure brake light switch in trucks).
The line port on your master cylinder "outlet nut" should be 7/16" - 24 int. thread. You haven't indicated what line nut is currently on the line pointing towards the master cylinder, but did say it's "too small". Perhaps the PO used a 3/8" - 24 int. thread nut when line fabricating?
You can get a brass adapter fitting to connect the 3/8" nut with the 7/16" port like
this from inlinetube, although I don't think the length of the adapter may be enough to fill the distance from the master cylinder to the line nut.
Alternatively, I think you should be able to easily remake the main master cylinder line with the correct length and fittings to get things connected and brakes one step closer to operating.
Sorry again for pointing you in the wrong direction and hope this gets you back on track.
PS - I've added an edit note to my post above to line out the bad poop indicated.
