The factory didnt use primer,so I dont either. As mentioned prep is key. I spray the block with brake cleaner,let dry,before I paint.
I use Mopar Performance Street Hemi orange paint.
That is the correct color for a 71 340.
IMO it is the closest color match to the original. Some will tell you it
is not that great of paint,I call it operator air. I have never had any issues with this paint, and it is readily available.