the 6.8 spc rim is smaller than the 7.62x39. the one im holding is .416. my book shows the 30 remington .422 so my round is off a little most 308 and 30-06 stuff I actualy measure is. the best bet for a 6.8 would be taking a 5.45 bolt and opening it up. It might actualy work with the x39 bolt as is or with a little work to the extractor or the extractor bore. the round will snap in but very loosely if the extracter was ground to let mive in slightly it might work. another member reported doing this on a 7.62x25 build and making it wok. If i was going to try it Id be sure I had a 5.45 bolt with the same head space incase it didnt work. I have a 6.8 barrel but I havent been real motivated to do it its a stainless heavy profile rmeinton 700 barrel. also there is a 6.8 spc II out now that uses hotter ammo and a differant twist.
the 30 remington has a case length of 2.05" so its .005 shorter than a 308 winchester. doing a 30 remington is doable but the brass is going to be expensive and it wont offer any thing special. If the bolt face that your worried about a 7.62x39 can be easly opend up in a lathe or even a drill press with a end mill. the mags would fit there are lots of take of barrels in 308 or 243 or 7mm-08, 300 savage 22-250 ect. the 300 savaage is one that if brass was less expensive would IMOO be a great round for a conversion. factory loads are lighter then 308 win and might be safe in a conversion.
if kits hadent gone through the roof I was going to do some home grown proof testing on a 2 lug trunion my self. a simple build with just barrel no gas system and a reciver to hold the parts.
Now the 35 remington is a idea I like and offers some thing that not a lot of other rounds do . but you will have to open up the bolt face. the 358 winchester based on a blown out 308 is about the same thing and could be done as well. a 308 mag would work well likely for both. If it were me Id go with the 458 winchester because surpluss 7.62x51 millitary brass could be to make it much cheaper and that brass would be thicker. now if you have a bunch of 35 brass then go for it. I have been thinking about a .35 cal pistol for a while for hunting my self. the recoil will be getting pretty heavy
sorry I confused the 30 rem with the 30 RAR.
extending the chamber on a yugo has some issues because the shank is pretty short and the chamber would actualy extend into the thined part of the barrel. it might be ok it might not. Im not sure how thick a 308 or 30 remington has to be at the chamber. the other issue is that the yugo has a .311 bore and the 30 rem or other uS rounds are ,308. accuracy would suck. you can do a a build on a cheap take of barrel rechamberd . If you want to do a build PM I can possably help you with the barrel work and the bolt work. the main thing is keep the pressures in a safe range. If your going to play with this stuff I cant recomend hand loading enough so you can control the pressures.