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This question strikes me as odd, simply because even if you have two bone stock Marui gearboxes side-by-side, the shimming setups between the two gearboxes will still be different.
There is no general setup.
If it helps answer your question(s), I've installed Prometheus bushings, Prometheus bearings, Systema bushings and Guarder bushings in to M14s. All of them fit properly, not too loose, not too tight, no mods required. All required re-shimming, although from what I recall, the Guarder ones required the least amount of reshimming.
If it's worth anything: I don't like the Guarder bushings, because they are prone to oxidation (read: rust) if they're not lubricated well.
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