1937 Pontiac, 223 engine, Oil Galley Threads?

[kookie1]

Can someone tell me what the threads are on the oil galley plugs of a straight 6, 223 engine, 1937 Pontiac. I tried to remove one the other day to measure it but couldn't break any of the three loose. By measuring the o.d. of the threads sticking out of the block it looks like 7/16" and a fine thread as well which is confusing. It's too cold in Minnesota right now to do any more but I wanted to order flare fittings for a hard line, remote oil filter.

After some research I see most people recommend using -8an flare fittings or larger with the usual rubber hose approach. I want to install a hard line instead. I thought if I knew the oil galley plug threads then the fittings for that thread size would determine the largest size tubing to use for the hard line. My previous thought was to use 3/8 npt threads but they are too large. Also would 1/4" lines be too small to use? Any help would be appreciated. Thanks!