The bolt and trunion assembly do nearly all of the work. The receiver just holds things like the buttstock on, provides a place to mount the bolt rails, provides a place for the bolt carrier to do its work and provides a place to pin the actuating mechanism. (trigger group)
The receiver also holds the trunion, but not a great deal of stress is transferred to the receiver. If there was a lot of stress those little rivits would shear off. I mean sure there is some stress but an AK receiver is not doing the same job a receiver on say like a Remington M700 does.
The 223 trunion fits the rest of the 223 parts, mainly the BOLT. All the fitting that is needed is to get the trunion in the receiver. Minor filing and fitting.
However, the tricky part is getting everything square and parallel with the receiver. But that is something fat peasant women with presses and moustaches have been doing for 50 years.
Mike