Good morning Marcelino!
I admire both that you've created the cluster insert from scratch, and that you'll wind up with an all-metal cluster housing! Your fabrication skills look fantastic. This way it's really "all yours."
Regarding the necessity to paint it, perhaps it's for the best. All of these bright-looking metal gauge housings for sale (like the one I bought ready-made) are cool-looking, but they're probably less functional than a cluster background that's painted a dark color. I can imagine that in many different driving conditions, the bright metal background will reflect sunlight in to your eyes and could be distracting and/or dangerous. The ideal background's probably a dark color that minimizes glare, and calls attention to the gauges themselves.
Have you already decided on a paint color?
When it comes time to wire your gauges in, since both of our trucks are 1972 models, perhaps you'd find the diagrams and notes of interest in
Could I ask for advice on attaching my Teleflex gauges?
I look forward to following your progress Marcelino, and congratulations on the one-of-a-kind job you've done so far!
Robroy