Now this is not from personal experience but I heard that any vac modulator you try should say for turbo engines. This trans was put in N/A cars as well so they made two versions of vac advances.
If yours does not work try this one.
http://www.transmissionpartsusa.com/Nissan_L3N71B_L4N71B_Turbo_modulator_1971_on_p/820-000082885.htm