piston questions

How inaccurate are the advertised CR on the magnums? If they're like the 70s/80s engines the CR advertised in all the manuals of the day are very ambitious. My last complete rebuild was a /6 and though advertised as 8.4 it measured out well below 8 to 1. I've seen small blocks that actually measured out as less than advertised which means less power than advertised as well.
Are these engines really 9:1 as built, stock from the factory? I want at least that much, but probably a max of around 9.5:1 so I don't have to run premium.
What do you think about running those pistons my kid has, now that I know what number they really are? The truck is an 80s which were still LA engines, basically I'd be building an LA in the confined of a magnum block ..
They said the magnum block is slightly shorter than the LA deck height wise, these 405 pistons are about 20 thou shorter compression height vs the 116s I thought they were. And the 405# are likewise flat top except they have 4 valve reliefs instead of the 2 that the hp116s have.
Even if not needed for a big cam, I would wonder what the valve reliefs did for mixture turbulence, when running especially the 2 that aren't gonna be used for valve clearance in each cylinder?