I always add drain petcocks to my Opel radiators and also to the block. Makes it a LOT easier. I recommend it to everyone.
Regarding flushing the system, I'd also spend some time flushing the heater core. Disconnect the heater hoses and run some clean water through BOTH hoses to flush and backflush thoroughly. If the radiator is really bad, I'd consider having it professionally cleaned. Blocked passages don't cool well!
Bob