Rudy, I believe 5/8" is what your car came with, but it may now have a mix of 5/8" and 3/4" due to the passing decades of folks using what was available. It is easy enough to use plumbing adapters to step-up or step-down hose sizes, as needed. For example, if the heater core is 5/8", and all else is 3/4", use adapters to connect to the core (especially since the core is the most expensive part in the heater system). The WP manifold fitting may be easier to just switch out to matching size; they are cheap, and screw out & in. If you need to replace that nipple, consider an 'L' shaped one, as used on later Stude Hawks, for reasons already mentioned above.
In a pinch, a 5/8" hose can be used on a 3/4 nipple, or visa-versa, but consider that only a temp measure.
In a pinch, a 5/8" hose can be used on a 3/4 nipple, or visa-versa, but consider that only a temp measure.
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