Is air drag the same on a tilted surface?

there are two things to consider:

- the tilt necessarily changes the cross-sectional ara perpendicular to the free stream; specifically it increases it. on that basis alone, if the drag coefficient doesn't change, then drag should increase.

- if the shape is more aerodynamic, then that drag reduction will be reflected in a change in its drag coefficient.

if i recall correctly, the drag coefficient for a bluff body is usually 1. so technically that should cover both the block and the tilted block, with the change in momentum flux being accounted for by the change in cross-sectional area relative to the free stream.

however, you have to be careful in that there are drag coefficients published for diamond shaped wing sections -- so depending on the aspect ratio of your block, you might need to use the diamond shaped airfoil parameters rather than the bluff body parameters.

based on the parameters you've provided i'm inclined to go with the bluff body approach. however, it's always best to run the analysis with both sets of parameters; then at least you can definitively demonstrate that there is "no appreciable change in results" when the underlying model is changed.