Okay so this is a very noob question I know and I searched and no luck. I have a 87 Firebird that came with the 2.8 but I did a swap to the 3.1 with no A/C. My heater isn't working and to my knowledge when I bought the car it wasn't working then. What could be the problem? Any help would be greatly appreciated. It's getting cold here!

 

Since under the hood may not be the same with engine swap, some components may not be there.
I would check the following:
Thermostat stuck open, water control valve gone bad, inop temp door in HVAC system in dash, clogged heater core, vac line missing/broken from water control valve...
Simply feeling the hoses to the heater core will give you a good idea if you have coolant flow. One of those infrared thermometers are great for pinpointing temperature differences. Looks like you got a warm up this week, so maybe not as important anymore.
Corky