Lemonade 2600i 4x4

So this 89' 2600i g6 has really given me a run for my money.
overheating
coolant blow off inside cab, white clouds
new head new valves new gaskets
over heating
napa heater core doesn't fit, off angle 2 degrees
thinking of going electric element and totally bypass heater core
any ideas

Radiator?
Thermostat?
Water pump?

I'd test for water pump and radiator first...

You're heater core may be leaking, there are a couple of small hoses on it which could fail or it has failed in some other fashion. Check your passenger floor for dampness.

I have some good used OEM heater sores if you determine that is the problem.

Dan