High Compression 390

Yes I read your post wrong, I did jump to open chamber heads, my bad.
I'm saying the magic of 185 psi and tight Q is phenomenal at 1000 ft, at 7000 it would be just as phenomenal, but the Scr to achieve it will be far greater than at 1000, so I think you're jumping the gun about lowering your STATIC compression. At altitude the TIGHT-Q will be even more important.
Now
The 390 at 3.94x4.00 is 799.177swept
To make 185psi at 5000ft, with a small-cam ICA of say 56* requires an Scr of 11.5.. A bigger cam would require even more Scr.... to make 185psi.
To make 11.5 with the above swept requires a total chamber volume of 799.177/10.5=76.11.
Ok then, 76.11 less 63+8.8 =4.3 in the decks and piston tops. If the pistons have 5cc pockets, then you are already .7 cc in deficit and .010 decks will add another 2cc putting you at 2.7cc deficit. You can't afford to give up 2 more in the heads.
But if the pistons come in at .010 down then you can run the .028(6.3cc) gasket, for a total of .038Q. This will get your total chamber volume back down to 63+6.3+5+2=76.3(with pistons .010 down) and now you are in the ballpark with no deckwork. (799.177+76.3)/76.3=11.47, close enough.
Now as to the 58* ICA, this is a pretty small cam. It could be something like 258/266/110 in at 109. Could be. With a 390 at 185 psi, the Wallace say it makes a VP of 181.That is really really big; so it's a torque monster. You will never ever need the granny gear. After the first time you stuff it in there, just to see, the only time you will ever use it again is for parades.Because 800 rpm will be about 2.8 mph.
OK now at 9000 ft your cylinder pressure might drop to 158 psi, and the VP to 158, which would be normal at sealevel with iron heads.
The beauty of this combo, is that if you want more power, you can slam any cam in there you want to, with a corresponding loss of pressure. But at 5000 ft this is no big deal cuz you have so much of it! of course at 9000 ft, the picture is a lil less rosy.
Ok so say you wanted to get 30 more hp at 5000ft. This is just about 2 cam sizes away, so lets work it out with a 272/280/110. In at 108, the ICA would be 64*. Your pressure would fall to maybe 171psi and VP is still 157; so at 5000 ft you still have monster low-rpm power, and now high-rpm power as well.
Unfortunately at 9000, your pressure is down to 150 and VP down to 138. However this still doesn't make it a dog. 138VP is about what a stock 360 Magnum would make at 1000ft.And your 272 cam is still gonna make plenty of power once winding up...... which you will need with that trans,lol.
One more thing; with the bigger cam, at 1000ft the pressure rises to 193/177VP, so you can still run WOT down there on pump gas. And with a VP of 177now, you can twist your driveshaft up pulling semis out of the ditch.

So bottom line I think is to get the short block built and see what the deck heights come in at, before worrying about the 2 cc in the head you are asking about. As long as you stay with aluminum heads, you cannot, I say cannot, go wrong.

This is an excellent engine combo with a lot of adjustability built right in, at an affordable entry point.