Originally Posted by MechoShade
I've also discovered 2 pools of coolant. One right next to the valve cover and other near block (both on left side on engine)
Looking at where your coolant leak is and your description of idle problems, I'd be looking for a leak around the intake manifold. A poor fitting gasket, would explain both problems.
Another potential leak area is the small bypass hose, on the bottom of the water manifold tube. Hard to see from above, unless you know where it is.
Your engine temperature fluctuation makes no sense. The opposite should happen: cooler running, with the heater fan on. Check your electrical grounds. This sounds like a ground feedback issue and not mechanically related.
The original stock Hitachi carb did not come with a manual choke. Do you know what kind of carb is on there now? A picture of that, would be very helpful in figuring out your mixture problems, too.
Leaves and crap on top of the heater core are very common. Do you have any airflow out the bottom vents, when the dash control is in the floor position? If the heater core is covered in debris, you'll get nothing.
There are some ways to clean off the core, without tearing everything apart to get to it. Do a search and you'll find a method, that involves cutting an access hole in the side of the heater box.
I've had some success with a leaf blower, aimed at the floor vents. Put the control in the dash vent/floor position, fan off. It will blow the crap on top of the core into the dash vent tubes. You can then put the control to dash only, turn on the fan and crap will blow out the dash vents. If you leave the control on floor, it just blows the stuff back onto the heater core. It takes several cycles, but it will work, if the stuff on the core hasn't molded into a solid mass.
By the way, your picture is of the right side (passenger, here in the USA).