I'm not sure about a few things like if these lines were original or not, but the last 1972 C10 that I changed brakes on, both rear and the right front hard line inverted flare nuts were 3/8 -24, but the left front was 7/16 -24. The flex brake hoses that came with the kit were all 3/8-24 so I used a
Dorman 785-434 line adapter to easily make things work without needing to re-flare any hard lines.
https://www.summitracing.com/parts/rnb-785-434