Hmmmm? I've installed a couple of those block heaters and never followed Diesel giant's advice. It seems VERY unlikely to me that there is going to be an air bubble in the lower radiator hose. The air will rise up to the top of the system.
If you're worried, loosen the top fitting of the lower radiator hose and make sure coolant drips out. If it does, the heater should be sitting in coolant and it's ok to fire it up.
Or, wait for spring.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13