Re: Under dash heaterbox.
I can only tell you for how I found my 67 heater boxes. They had a slight texture with a semigloss finish to them. I did a long search for the paint and found one that had the exact texture, and it also is the same texture as the top of the dash. It's Rustoleum 'Fine Texture" P/N 7220. I used this on the heater box and it also gives the semi-gloss finish. Use light coats over an already painted black base and just enough for the desired texture. I used this also on my dash top and it came out perfect (when a coat of flat black was applied over the textured finish). I compared that to my other original 67 and the textures are the same.
Hope this helps,
67 LOS SS/RS L35 Hardtop - Original w/UOIT
67 NOR SS/RS L35 Convertible - Restored