I've also been thiking about his combo for a couple of years. Search 6.8 Forums under the alternate AR calibers page. There are some guys trying to build a couple.
The big thing is being able to find a barrel. You'll have to get one custom built and all the builders out there are focused on .308, 5.56, 6.5, and other calibers right now. You'd be on the bottom of the list. I'd love to build a 20 inch barreled .358 on a Mega Arms MATEN upper. As for the gas port size you could probably have one drilled slightly larger than "standard" and buy the Kies adjustable gas bock. It works great on my .308 MATEN.
