BEFORE TAKEOFF, THE PLT WAS BRIEFED ON THE WX, INCLUDING SIGMET 20C, WHICH CALLED FOR IMBEDDED THUNDERSTORMS. WHILE EN ROUTE, HE DEVIATED AROUND AN AREA OF WX & WAS PROVIDED VECTORS TO AVOID STORM CELLS. IN THE VICINITY OF DAISETTA, TX, THE CONTROLLER SUGGESTED A FURTHER DEVIATION TO THE NORTH DUE TO WX. THE PLT RESPONDED THAT A NORTH TURN WOULD TAKE HIM INTO A CB TYPE CLOUD. HE DECLINED THE TURN & CONTINUED ON HIS HEADING. ABOUT 10 MI LATER, WHILE CRUISING AT 8000 FT, HE ENCOUNTERED HEAVY RAIN & A VIOLENT DOWNDRAFT & LOST MORE THAN 3500 FT IN A SHORT TIME. DURING THE ENCOUNTER, THE ENG COWLING SEPARATED & KNOCKED OUT THE WINDSHIELD. THE PRIMARY FLT INSTRUMENTS BECAME INOP & EXTENSIVE DEFORMATION OF THE AIRFRAME WAS SUSTAINED. THE PLT SUBSEQUENTLY RECOVERED IN VFR CONDITIONS BELOW THE CLOUDS &MADE AN EMERGENCY LANDING AT CLEVELAND, TX.