Unfortunately John, you're in the States and I can only provide you with Australian solutions. A number of years ago, I had a local company in Sydney make up some Head and Rocker (Tappet) cover gaskets for both 6.3s and 6.9s. The cost was way, way below from the original price quoted by MB in Australia. The rocker cover gaskets were a simple "hard" copy of the originals from MB - simply smear them with sealant and you won't have a leak. My job was done ten years ago, and certainly leak free from any oil seepage.
The 6.3, unfortunately, is a bit different. Due to their design, you really have no choice but to go genuine. They are made of rubber, and I don't even know if MB still stock them. I've personally done no research, but you are an old hand and I trust that you will be able to direct enquiries to the right people..
Head Gaskets - yes, I had some of those made up. With the 6.3 units, they have an outlay of copper materials, and one may lose a little by way of compression on account of the increase in thickness size of the gaskets. I relation to the 6.9 gaskets they are very close in specification to the originals, and no compression is lost - certainly, as best as I can tell. Those jobs were also done some ten years or so ago, and there have been no water nor oil leaks. It's a big job - not for the faint hearted. BTW, why are you asking those questions ? Regards Styria