Master cylinder brake line replacement

dustymopar,
Sorry I don't know the size of the nut used with the 1/4" tube, but it is the smallest size that fits that tube.

None2Slow,
I am currently installing the dual master in my 64 Valiant, so will try to post photos. Actually, I am working on my house, but ...
The link that VDART gives is great and they do say 7/16" thread for the top 1/4" port of the splitter block, so should be the same on the original MC side. They don't give PN's, but I got them at Autozone on the brake fitting spinner display.