Not impressed with the heater core I bought from Advance Auto. The tubes weren't bent correctly. I had to oval out the holes in the sheet metal plate that is screwed to the box. I also had to trim the mounting flange for it to fit inside the box.
It might be a good idea to see if a radiator shop can fix your original one.
Definitely get a heater box gasket kit from DMT and install it while everything is apart.
I drilled a small hole in the thermostat to prevent an air lock. I was able to completely fill the system without the engine running.
