MY 66 Chevy Cooling system flush questions

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Now that I got my truck running time to do some maintenance. Ive had it around 2 years and the only thing Ive done to the cooling system was change a sticky thermostat with a new NAPA thermostat that still stuck just as bad.

There is no rust that I know of in the antifreeze its self, mainly just mineral deposits.
Id assume it is from someone using a garden hose for the water source.

What would you recomend for a system flush?The petcock is froze up on the radiator so I will probably pull the lower hose to drain it.
I would like to flush the radiator and the block, so could I just pull the upper and lower hoses, stuff a garden hose in the upper hose and blast it?
Could I do the same in the thermostat housing in the intake?
While Im at it I might as well change the thermostat again, What is the best temperature to run?
How many gallons of anti freeze will it take?
Im sure the radiator is the original Harrison rad. cooling a 350 block stroked out
I have 4 gallons of Prestone 50/50 mix and Im hoping it willl be enough.

Thanks

Thanks