1990 350 Express Cruiser Stuffing Boxes

Looking for the following info if anyone can provide? Prelininary questions:
1/ How do I determine what the diameter is of the packing I will require? I guess I will need to know my straight shafts diameter - correct? And if so, can this be determined based on my year and model or perhaps on the transmissions specifics?
2/ Once determined, what is the best material for the packing material and can anyone recommend a specific manufacturer / product?
Thanks to all!

 

Thank you for the above. I have always thought that a drip was required for lubrication but in reading about this product I am led to believe the stuffing is such that is is made of a material which in itself is infused to be already somewhat "lubricated" so water passing through is unnecessary - is this a correct assumption on my part?