Need help, advice with my 318 build

You are overthinking things.
Your Dcr will be fine with quench equal to headgasket thickness.Just put it all together as you said in post one. Since you have the 250 cam,plug it in. Put enough spring in it so it revs decently.If you have room for 1.6 arms do it.Put the cam in so it's exactly on split overlap or a couple of degrees advanced to exact split overlap. Forget about retard.
With that size cam, almost anything you do to the heads will help when the cam starts choking things up..If you are financially able, I'd agree on a bigger cam being ok, so long as it has a fast rate of lift. In fact, I think your combo would be a very good candidate for a custom cam.
The 2.76s, even with a 2.74 first may be a real downer, but you can deal with that later.If your TC is a little loose, it may be fine. For sure you are gonna need a SureGrip or two big tall fat tires.
You don't say where you are in Sweden, and since the elevation in your country varies to nearly 7000 ft, there may be plenty of headroom in the Dcr. I would slam it together with .039ers at Zero deck and tune the snot out of it.Keep in mind that; higher-number gears, less loading, higher elevations, looser TC, reduced combustion chamber temps, and later or lower full power-timing, are all tools to be exploited with engines running on the edge of detonation.There is no command that the engine must receive full timing at any particular rpm. Nor is there a command that it must have a particular amount of power-timing. Let your combo tell you what it likes, and tune it from there, using the tools.
For your stated use sacrificing compression in the hopes of side-stepping a possible WOT problem, when none yet exists, is to sacrifice some part throttle driving pleasure' and possible economy.Yeah I know nobody likes to build their stuff twice, but with your stated goals and with your chosen combo, I personally, would be prepared to.Besides, when it's all done and behaving like a terror, and doing all the things you wanted it to, then you can report back with your personal success story.
And if you do have to mess with Dcr after it's built, for goodness sake don't retard the cam. Get a cam with a later closing intake, or machine the pistons as already mentioned.Just don't mess with the quench/squish pads.
BTW the new tranny ratios are a wide ratio set, with bigger cams,they work best with a wider LDA. At 250, this won't be an issue. As others have recommended bigger cams, this is something to keep in mind . However I can't see it being an issue for you, with your stated goals
Ok so that's my 2cents.