If you want instant heat, install a multifuel burning heater (Stewart Warner South Wind), it is very effective, the TM even warns of warming the cab first so that the otherwise cold windshields don't crack from the exposure to hot air.
The hotwater personnel heater wasn't ever used on the 757, the South Wind does it all. Just another one of those little improvements I have posted from time to time.
It runs completely independent of the engine and is started first. It will run down the batteries after a few hours, so the engine should be started rather soon after things get warmed up.
I love my heater and if I had the funds, I'd install one in the deuce as well. The hot water (coolant) version takes too long to be effective....and when the engine gets warmed up the heater isn't needed any longer....