One would assume a new sending unit is working OK. The best way to check is to check the resistance using a multi-meter. I don't know if 72 still used the 30 ohm sending unit/gauge set up or not. A check of your shop manual should tell you that. Assuming 30 ohms is correct and using 30 ohms as an example, if the sending unit reads 30 ohms with the float in the "full" position, the gauge should read full. If it doesn't and all connections and grounds are good the gauge would be at fault.
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