We extend to you a cordial "WELCOME ABOARD !" Come on in, make yourself at home, we are a friendly group of enthusiasts, and we also appreciate the classic Chris Craft Roamer, Corsair, and Lancer boats too , as they are all on the same family tree and share much in common !
Size of packing is that size that you can stuff into the packing gland and overlap the joints. If it is too skimpy it's not as good as if it is a nice firm fit. Shafts and packing glands vary, so it's good you got two sizes to work with.
The "white rope" is the type of packing that should be used, no problem there. It's not necessary to remove all the old packing, as it tends to wear out with time and tightening. The important thing is to get enough packing in place to allow the gland to be tightened up to a point where it will drip slowly under way. You want some dripping to keep the packing from getting too hot. Newer teflon packing helps. As for the packing itself, helping with vibration, that is pure "BS" in my book. If you have a mechanical misalignment, which I suspect you may, putting new packing in isnt going to do a thing for you, again IMHO.
If you can rattle the shaft while undewater, I guess you have a bearing that needs to be replaced.
As for the price, I think you need to find another marina.
One last comment: be sure you insert a long and large stainless cotter key through the open hole in your brass packing gland, and run it through a corresponding hole on the backer plate. If these packing glands get dry, they want to rotate and you could rotate one to the point where it opens up to incoming water like your photo shows! Regards,