The block should be perpendicular so it is not along the spine. Instead, you can bet both shoulder blades on it. And, I use the low side of the block, meaning at its lowest height. It's really simple stretch and should open up the front of your chest.
https://www.youtube.com/watch?v=QJk_F1ZX0xo&list=PLurBO9DSc13Hy7tFKIJA3Vlh7R_ovxZZ2&index=1 Do this on non-surfing days. After two weeks I can definitely can feel a huge difference in how loose my arms and legs are.
I've been doing DDP Yoga for about 4 months now. It helps immensely. Sure, it isn't paddling or surfing. But both have improved since I've been doing it. Snowboarding too.