What is the flushing history of your block and head? When you open the tiny drain cock on the lower back driver side of the block does nice clean coolant flow out? The hole in the thermostat is a good idea. I put in 3/16 holes in mine. I recently lost a head on a gmc 228. When I pulled it, I foung that most of the coolant nozzles at the head gasket surface were plugged off with rust and scale. Mineral deposits come with the tap water you add to the system. Use of distilled water can help with that. Modern coolants have corrosion inhibitors. If you are running less coolant with tap water, the job may not be getting done.
I would drain all your coolant into a clean container and examine it. Then I would pull the thermostat, bypass the heater with a crimped hose looped back to the connections at the water pump, install a 1/4" full port ball valve with a nipple replacing the drain cock at the back of the block (you can add a 90 on that so that you can drain down into a container.) Then do a flush with chemicals designed to break down the scale and rust. The factory shop manual has pretty good descriptions of the cooling system and flushing instructions.
