"Factory Original" is generally parked after assembly, trunnions riveted in place. The receiver remnants removed from original used kits reveal bare metal between the receiver and trunnion, often with light rust. Consequently many people park their receiver shell and attachable parts before assembly, then redo it after riveting. Bbl in or out is dependant on whether your container is long enough.
Preserving the original finish? Why? I'd rather have a nice modern finish than whatever cheap paint the factory used.
Prep for park is essentially disassemble as far as possible without removing pressed on parts, removing springs and anything the abrasive will stick in or jam up. Degrease and blast clean, degrease again if needed and dunk. If you are re-doing the park after assembly and not painting over it, wait to oil it up until finished.
As for blueing, check out Valmet's Rust Blue thread over on the AKFiles, he is getting great results. A bit labor intensive but the results are phenominal!