block heater

Anyone installed a block heater in a 420A?  While I know the anitfreeze is supposed to be good down to -30, it has been getting into the minus teens around here lately and I want to really be sure I don;t blow a freeze plug or worse, a block.  Any recommendations on type would also be appreciated.

I think that they make some that you can just put in the dip stick hole. Try looking for those.