Congrats, a very nice NCC 505! Seems to be in good condition and doesn´t look bad on the leather strap with Rado close.
The same case design is available in tungstencarbide on the DiaStar 515 and one of the Certina DiaMasters(which is the same with Certina branding).

A silver coloured watch, polished to gold plated? A mystery.
These are all SS-cases, so chromed brass isn´t an option. Also the GP ones. So polishing the gold plating away and have a SS case is easy to understand. But the other way round?
Markers, hands and the anchor of the moving anchor logo are golden, so everything is alright. What´s the reference no.?

I would leave it as it is(and replace the SS crown by a GP one). As we know meanwhile, it´s an authentic GP model with the correct reference no. and it wears the correct dial/hands-combo with golden marker-ring, anchor and hands. A SS-case would make a Franken out of it.
If you absolutely want to have a silver case, it doesn´t make sense to plate it again - the case is SS under the GP, so all you have to do is polishing, polishing, polishing and soon the GP will disappear.
A standard polish for SS and/or chrome(cars) will do the job. You shouldn´t use machinery to keep the edges sharp. But IMHO it would be a shame. Maybe selling the GP one for a good price and buying a SS model would be the better way.

Here´s a pic of my Certina DiaMaster(Rado DiaStar 505) with the same case design, but TC-case:

These references only:
NCC 505 - 11942/625.3037.4. - 1970/1978 - 15.000 - SS
NCC 505 - 11943/625.3037.2. - 1970/1978 - 7.000 - GP
NCC 505 - -/625.3174.2 - 1976 /1979 - 3.000 -GP
NCC 505 - -/625.3174.4 - 1976/1979 - 2.000 - SS
AFAIK these cases are all stainless steel, also the GP ones - but I wouldn´t cut one to be sure.
What you describe sounds like brass, indeed. Brass is more yellowish than GP - is there a difference in the colour compared with the GP close of the Rado strap?