How about adding a coolant recovery bottle like on a newer vehicle?
Any coolant that burps out gets returned to the system, this way you can keep the top tank full and add some capacity.
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1964 Tempest Coupe LS3/4L70E/3.42
1964 Le Mans Convertible 421 HO/TH350/2.56
2002 WS6 Convertible LS1/4L60E/3.23
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