Great idea to do a 98mm cylinder, similar to those done back in the day by ottinger (sp?), but to encapsulate the head studs to provide additional rigidity, especially if you get the desire to put a turbo on there. The only thing I will be curious to see is how well they cool, since we actually de-encapsulated the head studs on all our 930 cylinders to provide better and more even cooling. It was only with the 993 (3.6,3.8) that they absolutely have to be encapsulated, as there just isn't enough meat to seal them off otherwise. Are you going to go one step futher and flame ring the cylinders and heads, to ensure proper sealing. One could easily use some from 3.4 cylinders and adapt them to work. They would be overkill with the registers you've used :-)
Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance