It's cold..... Really cold.

[JThiessen]

Just being enclosed will make a huge difference - especially if there is any wind.
Like stated earlier, just leave a couple corners open for a little air flow. At the temps you have right now, you can get away with complete coverage.
If you have a block heater, Dodge sometimes had them coiled up on the passenger side of the radiator (I'm assuming Dodge since you said 5.2 Magnum). Put your card board on the inside if you have to.