1980 C10 Inline6 250 Carb vacuum and brake booster

[C10wip]

I have a 3 spd manual, and I pulled the plug I'm pointing to and hooked the booster there. It's 3/8" NPT thd on the manifold. In your case I think the line that is screwed into it now goes to the automatic transmission and it looks like there is a bushing screwed into the manifold and the trans line is screwed into that. Maybe you can add a tee in there somewhere. Whether or not that would affect the transmission, I don't know.