You obviously have extremely limited knowledge of cars in general.
Flushing the cooling system - Buy Tectaloy Mukowt - drain green coolant - add Mukowt to header tank and fill with clean water only - drive the car for an hour or two - remove bottom hose/drain coolant - remove thermostat - remove top rad hose - reverse flush the block from the bottom hose until the water runs clear.
Leaking head gaskets do not always over-pressurise the cooling system.
Just because the heads check out ok doesn't mean the head gaskets aren't compromised.......
Go to a workshop (ANY workshop) & ask for a TK test - it should be $15-$20.......... It will tell you if the head gaskets are fucked or not.
Why ask questions if you're not gong to heed the advice given?
Bolting a whopping great turbo on a motor that has a non-tunable
ecu is extremely stupid.
Bolting on a whopping great turbo to a stock motor that is already showing signs of being fucked is even more stupid.
Buying a jap import motor & expecting to be able to run that big turbo reliably as your back up plan is stupid.
Meh............You just can't help some people...............I'm done here.