While chili powder can provide some similar flavors, it may not be the ideal substitute for Guajillo and Pasilla chilies in an Al pastor marinade. Guajillo and Pasilla chilies have distinct flavors and contribute to the authentic taste of Al pastor. If you use chili powder as a substitute, the flavor profile of the marinade will be different. However, if you don't have access to Guajillo and Pasilla chilies, you can try using a combination of chili powder, paprika, and other spices to approximate the flavors as closely as possible. Experiment with different proportions to achieve the desired taste in your Al pastor marinade.