Block heaters are common place/manditory on some vehicles up here (any vehicle will accept the right sized block heater in the correct location on the block) that's why I suggested it, you can install one or a frost plug with skid plates off easy if you don't knock the frost plug inside the block by accident trying to remove it. I never installed one on the 99 just the 97 plow truck, 5w30 syn and see -35*, the 99 5vz-Fe starts every time, and I rarely plug in the 97. Those block heaters have a oring of unknown materials or lifespan so they can degrade and cause leaks and a blowout.

Glad you caught it before you got into the red, I lost a lower rad hose wheeling and developed a minor head gasket leak in one cylinder, replaced the motor..