5.9, max safe compression?

IMO, 8.7@ sealevel is too high for iron heads, but I have not seen enough data on this site to say that for sure. 8.7 will be easy for aluminum heads tho. The best guys to talk to would probably be the guys who build them.
One problem I foresee is that with .040 quench you have nowhere to go with a thicker gasket, to reduce the total chamber volume. Adding just .010 to the gasket, will put the Q into the gray area of .050 to .080. And that .010 gasket will add a little over 2cc to the total chamber volume. So the theory would say that the extra 2cc would bring the Dcr into the doable range, while the Q goes into the don't-do-it zone, and perhaps just makes it worse. So you add .010 more, and it gets still worse!. I'd hate to see you buying thicker gaskets every week!
Far better to engineer it right in the first place even if the Dcr ends up sub-optimum. Or you could just go with a later closing intake angle,lol. But swapping cams every week might get old too.
We know 8.2Dcr/iron works at sealevel. The difference from 8.2 to 8.7, may not be worth chasing, unless you got something else riding on this combo. And if it gets up there, we also know that aluminum works at 8.9 to maybe as high a 9.2 as some here on FABO have claimed.
It may be cheaper in the long run to zero deck it and run some closed chamber aluminums, as I did. By zero deck, this time, I mean running quench in the range of .035 to .045. That could mean a zero deck and up to a .045 gasket, or .028 gasket and up to .017 in the hole. etc. Then let the Scr be what it will be, and letting the Dcr fall into the range of doable; I run 87E10 on a Dcr of 8.7aluminum , IIRC,one fellow here runs 9.2Dcr-aluminum on 91.
>Bear in mind that your engine only needs enough octane to suppress detonation at maximum loading. Probably 80 to 95 % of the time in a street-engine's life, it will run just fine on 87. Yet we have to put the gas in the tank for the maximum loading. Cuz we just cannot control our right foots,lol.
So, if your engine gets into detonation with a 750DP, perhaps downsizing to a 600Vacuum secondary, or a 500AVS, will keep it out of detonation. Or my favorite trick is to use a spreadbore and slow the secondaries down, for the week. Then on the weekend, put in the hi-test and crank the secondaries back in! I mean the 5.9 will have no trouble spinning most street tires anyway, whether at Dcr of 8.7 or 8.2 or maybe even 7.7,lol.