One more thing to think about. If it still has the original steel shim head gasket,. It's possible that the water jacket restrictions are rotted out which let's all the water come up from the block at the front of the head instead of the rear. Since it sounds like you have an infrared temp gun, just check the temp front to back. If it's way cooler at the front it may need a new head gasket. Also I've had blocks so plugged up with sediment that a garden hose wouldn't flush it out. I used a 1-1/2" gas powered water pump with no tstat in and just kept swapping top to bottom rad hoses until the water came out clean. Don't be surprised if you have a rusted out freeze plug after you're done.
