1999 Oldsmobile 88 heater core

Minus 45!!! You're going to be disappointed. You are limited to engine coolant that is regulated at less than 200 degrees F. and you have a limited volume of flow through the heater core. The only variable is the temperature of the incoming air. If you're lucky, the heater core will raise that air temperature about 60 degrees, less if you run the fan on a higher speed. That means there's more cold air but the same heat volume. Don't expect to tear your coat off in the car when it's that cold outside.

Changing heater hoses isn't going to change anything. Those just deliver the hot coolant to the heater core. If anything would help, it would be to wrap insulation around those hoses, but I doubt you would notice any difference worth going through the work or expense.