Yes the a525 trannies are #'s matching to the VIN. VIN stamped on pad (see attachment). I have no idea who would accept a a525 core for a a555 build, as they're two completely different transmissions. IMO, if you are even slightly concerned about originality, and you're committed to upgrading to the a555, you should choose the rod shifted option for the a555 conversion. At least that way the shifter remains stock. Trying to mate a cable shifted shifter into a Omni console certainly is no bolt in. There are some amazingly hacked up attempts out there. IMO, even the "Omni from Hell" conversation left a lot to be desired, and that had factory ties.
Different starter, tranny, rear anti-rotation mount, axles, intermediate axle shaft, shifter mods (whether rod or cable), speedo gear, etc. need to be changed out. So hardly a standard bolt in. Better be handy with a MIG/TIG welder. The good news is the clutch and PP assembly is the same, until you get into the a523/568 series.