I've thought of a more elegant solution.
Ford, for sure, used a steel brake line nut that was for 3/16 tubing ... yet had an "oversize" 7/16-24 thread size, which is the exact same size as the "standard" 1/4" tube nut.
Cut off the original 3/16 tube nut and replace it with this "oversize" one. Now both nuts should thread directly into that inverted flare coupler.
(This picture may appear to be brass, but the description says "steel")
AGS # BLF11-C-5
Papco-Paulin (Canada only?) # DS141-4-3
NAPA Balkamp # 641-3296
A steel tube nut and a brass inverted-flare union is (to my knowledge) perfectly OK. No galvanic corrosion issues. Think about it ... tranny cooler fittings on an automatic transmission were brass (as they also were at the radiator) ... and they used 5/16" steel line. Fuel pumps also had brass fittings and steel lines.
I have also been told that the correct way to splice brake lines is to use double-flared lines and an inverted-flare union.
1000% better than a compression fitting!!!
(Yup, I have had customers tell me that they have made repairs to steel brake lines using compression fittings!)