I have a 6.8 spc but I don't have a mini 30 to compare to, it may be a thickness in the mag well that may keep you from getting this done. I will have have someone measure the mags of the mini 30, I will measure the 6.8 mag.
With a RCBS plastic caliber I measure a MINI-30 mag, 20rd as:
0.906" wide x 2.404" long.
I know the 7.62x39 case has more taper than a 6.8 SPC case.
I think 15 rds might fit if the follower angle is different.
Not sure as to the bolt stripping out a case to chamber.
I was considering rebarreling a Mini-30 to 6.8 SPC - SPEC II chamber with SPEC II twist.
There is energy advantage to the 6.8 over the 7.62x39, and maybe there will be less expensive 6.8 ammo coming out in 2013 by both Federal (American Eagle) and Winchester. I am supprise WOLF (Russian) was not picked up on it.