Leaking 727

Input shaft leaking is not that uncommon. It happens with age but if its a fairly fresh rebuild it may have been damaged when installed. Worn bushing in the pump that supports the converter snout will cause seal to leak. This should be replaced with a rebuild but not everyone replaces bushings when rebuilding.
Sometimes they can leak if the converter snout has a wear groove. Obviously wont be your problem with a new converter. Another possible source is pump bolt seals but these usually dont leak in my experience.