Actually there is a fill plug screwed into the top of the box for filling. It is a ford spec grease that comes in a tube, don't use lube it's too liquid and will settle. Here is a great description of how to fill it in the car. 2 people and a grease gun is easiest - 1 to hold the nozzle to the fill hole, one to pump the grease gun:
I looked it up on NPD before buying mine locally, it's Motorcraft NL G1 #2 it looks like:
I lent my tube to a buddy to fill his so I can't check mine but that's what I bought. It made a big difference in steering play as it did for my buddies '69 also.