FWIW, I'm doing the same (wx permitting - looks
at the mo) on Monday Malaga - Porto. Have been informed no customs/immigration requirements, as both Schengen countries. I assume you will be stopping for fuel/coffee somewhere in Schengenland (Spain, I imagine), in which same should apply.
Here the nos of the Portuguese Immigration Service:
T +351/217/115085
F +351/217/168064