Thanks for the replies guys. As for the heater core, I have no idea if it's aftermarket or not. My father in law ordered it, so I couldn't tell you. Plus if I asked him he probably wouldn't know or remember. And he would probably give me a little lip saying it doesn't make a difference. He's like that. What I do know is I have a very slow anti-freeze leak someplace. Because I find little green/yellowish spots in my yard from time to time. More easily seen in the snow. Where it's leaking from I don't know. But I recall it leaking when I bought the truck, and still leaking after I had the heater core replaced.
As for the temp gauge, it climbs and stays steady a little bit before the halfway mark on the gauge. So I'm assuming it's working correctly. I just wish I had a gauge that displayed the actual temp reading. I'm honestly thinking it's the blend door, but I'd rather not have to get the dash completely removed again to have to fix it. Already went down that route once with the heater core replacement. Luckily for me the mechanic I bought the truck from did the labor part of the job for free. A 7 hour job supposedly. So I lucked out there. But if I have to get the dash removed again to change or fix the blend door I don't think I'm going to get lucky twice.
Anyway, yesterday was a milder day out. So I got my wife to drive and I checked the heat distribution around the truck. Coming from the windshield defroster the heat was just warm. But when I put the heat on the windshield/floor the results were a little bit different. On the windshield the heat was still warm, but on the floor on the passenger side it felt hotter, (Strange). So I checked the floor vent on the drivers side and it felt cool, or slightly warm. But not as hot as the one on the passenger side. Right there I thought blend door. But i couldn't understand why the heat just on the windshield was just warm. Anyhow, if anyone can shed some more light on this situation with the information I just gave that would be great.