Technical Coolant blockage?

So I realized the other day that my temp gauge wasn't working (54 Dodge with a 241 Hemi). Decided it was time to fix that before I got her out on the road.
After letting things cool down, I popped the radiator cap (no pressure behind it) and then cracked the petcock to drain things down a bit.

When I went to take out the stock temp gauge, there was still quite a bit of pressure behind the probe (intake...driver's side).

Long story short, I finally got it out (and got water everywhere). I proceeded to remove all the hoses to the heater core, and plug stuff off. When I squeeze either radiator hose, water moves on both sides of the intake. I plan on pulling the radiator, and flushing the block thoroughly. Wondering if there might be another spot clogged up, or if the heater core was my most likely culprit.

 

I should add....the core was bone dry when I pulled hoses.