I come up with an expected 47 in^3 expansion which is 0.2 gallons. so one quart would go from empty to full, going 0.5 gallon would give some margin and still not be excessive. recall the existing is 0.7 gallons. If the car ever boils, then the extra catch volume would be nice ( I don't plan for boiling though)
remember, I am stating this from keyboard and don't have any hands on experance. so I don't mind being corrected if I have made an error
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