in search of warming my engine [83 MB 300sd] to start in the cold [i don't believe it has gotten above 35degress F in the last to week], i purchased the kats 200 watt magnum magnetic block heater. Well, actually i ordered the 300watt and was sent the 200watt.

before i go about ruining the packaging of the 200w or exchanging it for the 300w, i am curious if the 200w is strong enough to warm my engine block. Of course this depends on the outside temperature, the placement of the heater, and duration of use... nevertheless, i would like to get an idea of how many watts are needed. i understand that the factory block heater for mid 80's MBs runs at about 400watts.

should i buy two 200watt heaters or exchange for the 300watt? any other suggestions?