The trigger group and receiver of the AC556 are not interchangeable with the Mini14. Dimensions were altered to allow for the selector lever which is mounted on the rear of the receiver. However, some parts like the mag catch, mag catch spring, the hammer spring and hammer spring strut are all the same. When it comes to servicing, it's better to send an AC556 back to Ruger anyway. Regarding the barrel, that can be done by any gunsmith with the proper wrench. However, critical to reliability in full auto use of the gun is the gas port size as the shorter the barrel, the larger the port.
Speaking about making a receiver wrench, why can't one be made with a metal bar and JB Weld? The barrel block can be carved from maple (which is better than oak since it won't splinter as readily as oak will). I