Hi DCinCA, and RichardL (flattery appreciated BTW!)
The correct oil line for the plunger is the longest one piece pipe, with the paired rocker banjos on one end, and the banjo on the other end which is locked between the metal surfaces of the two fittings at the oil tank.
The metal to metal seal will work fine; when the banjo sits concentric between the two fittings, there is a point contact for 360 degrees, on each fitting. From memory (and I might be wrong about this) the tapers are 30 degrees.
Suggest you grab the fitting which screws into the tank in a vice. Set the proud tapered face that the oil line banjo sits on, facing upwards. Check the taper is perfectly smooth and no scratches , dings, etc on the sealing face. Give it a rub with very fine wet & dry.
Do the same checks on the male tapered face of the other (outer) fitting; the oil line pushes on to.
Then do a "dry" assembly and firmly tighten the outer fitting.
Disassemble and check for an uninterrupted line on each face, where the banjo end has seated on each face. The arrangement wont leak if you can see the continuous contact at each face.
Reassemble with a thin smear bearing of bearing (Prussian) blue, which will make it easier to see the metal to metal contact. Or, chalk or talcum powder.
Also, use a sealant (569 Loctite is perfect; with Primer T recommended) for the male thread on the oil tank, when screwing on the inner fitting.
Happy days, cobber!!
Richard