Why make this so difficult?
If the fitting going into the Carburetor is NPT, you can purchase the ferrule and nut to fit the elbow, as long as the ferrule and nut are the correct size for the line it makes no difference.
Any good hardware store should have copper tubing and the fittings.
Years ago, everything was flare fittings, that too has gone, many do not even have the knowledge or tools to flare tubing today.
Copper and brass have gone out of site in prices too.

If you do not have a thread pitch gauge and at least a good caliper, Dial is easier to read, you should invest in both.
CTS

Cats prices are out of orbit!!! and it's only going to get worse!!! They brag about preserving Cat History but put prices on old machine parts are way over reason.

9/16" x 18

the easy way do you have the ferule nut and the bowl take them into your local hose and fittings joint they will match up some thing suitable and possibly be 2 thirds the price.
but it doest really matter what thread the other end is provided you have a suitable flared end or nut and olive. its probably better to change it to something common than unuasaul as you can then get the parts if it needs replacing again.

If it helps, The angle on the original ferrule is 25 degrees on a side. 50 degrees total. These have a slight radius easily achieved on a lathe with a file after cutting the taper.

Neil: No promises, but I had a small container of fuel line parts and such at one point. I will look and see if I can find the items you are looking for. JM

if your female end of the street elbow is 9/16 -18 TPI that should be the same as #6 JIC. so you should be able to get a 1/8 npt male by #6 JIC female in a street elbow JIC is 37 not 50 but if I was betting I would say the 50 is really 45 deg witch would make it SAE #6. hope this helps mc master car should have them