Slice chiles in half lengthwise. Remove and discard stems and seeds. Place chiles in a bowl, and add boiling water to cover. Let stand 20 minutes or until chiles are softened. Drain chiles, reserving liquid.
Cook cumin seeds in a skillet over medium heat 30 seconds. Add oregano, and cook, stirring constantly, 30 seconds or until cumin is toasted.
Process cumin mixture, soaked chiles, 1 cup reserved liquid, garlic, and next 4 ingredients in a blender or food processor until smooth, adding more reserved liquid if needed.
Place pork in a shallow dish or heavy-duty zip-top plastic bag. Pour half of chile mixture over meat. Cover or seal, and chill for 30 minutes. Remove pork from marinade, discarding marinade.
Stir together sour cream and 1/2 cup Cucumber-Radish Salsa; cover and chill until ready to serve.
Grill pork, covered with grill lid, over medium-high heat (350° to 400°), turning occasionally and basting with reserved chile mixture, 20 minutes or until a meat thermometer inserted into thickest portion registers 160°. Remove from grill; let stand 10 minutes. Coarsely chop pork. Serve in warm tortillas with remaining Cucumber-Radish Salsa and sour cream mixture. Garnish, if desired.