Pookie there is vert little room between the rear of the carrier and the front of the rear trunnion when the carrier is in the full backstroke position, maybe an inch and a half. However, you have about 4 inches that can be bored out of the rear stock before you get into the void. I plan on making use of that.
One other thing I was looking at is that maybe you can use the PKM spring but just relocate it off to one side of the carrier and drill a hole through the rear trunnion and into the stock about 4 inches deep. The total compression length of the spring would only be reduced by 2 inches.