More than likely a 352". Not sure on the year, '65 and '66 trucks didn't change much. Might ask over on the Ford Truck Enthusiast Forum
http://www.ford-trucks.com/forums/forum40/. Bunch of Truck nuts over there.
Some of them even know what they're talking about...
Get the #'s off of the VIN plate. Maybe we can decipher what it is from that.
Sorry couldn't help more.