No heat coming from heater?

Hello, it sounds like you may have an air pocket stuck somewhere, or the heater core may have become clogged up with the gunk, I'll call it, that can build up in the system. Did you drain the entire system and when refilling did you go through any type of air bleeding procedure? I will post the service info on the 'Drain and Refill" section for you.

Just make sure to never open the cooling system when the vehicle has been running, you can be seriously burned, don't ask how I learned that hard lesson a long time ago.

These are the refill procedures, but really the best fastest and easiest way is to buy a vacuum cooling system refill kit, it works with shop air, pulls all the air out of the cooling system and holds a vacuum for a couple of minutes to verify there are no leaks, then you just put the hose from the unit into a gallon jug of coolant and the vacuum the system is under will just draw all the coolant right into the reservoir in just a couple minutes, with no air in the system at all. I will never go back to using any other method unless it's for some unknown reason.
This is the unit (3rd diagram).

Were you having any issues with heat previously to changing the water pump? You didn't accidentally break or knock off any vacuum lines to a diverter valve on the heater core hoses or anything like that?

With both hoses going to the firewall and into the heater core it sounds like some coolant is getting through, but that depends on how much you drove it before checking the temps of the inlet and outlet hoses. If the heater core is clogged, I clear them out by running water the opposite way through the heater core using a regular water hose but regulating the water pressure I apply to the heater core, Most Cooling systems are only designed to run 15-18psi max, I wouldn't even go that high when flushing a heater core, Just enough to see the gunk coming back out the opposite hose. Having both heater core hoses disconnected, and a piece of hose on the inlet side (which is now the outlet side when flushing) so you can catch all the stuff that comes out into a bucket.
Here is a video of a 2000-2001 Chevy, but these guys are not the best, just pay attention to the method, they're making a mess in the video. But dont push more than 10psi, so low pressure water through going reverse. If you have never done this before you will see a lot of garbage come out.
https://www.youtube.com/watch?v=VbExKegW19Y

That's great, what did you find was the cause?

Thats a very interesting and creative idea, I like it. Nice work.