One factor explaining why it's hard to duplicate the factory 10.5:1 compression may be that all the head gaskets available today have the large bore holes (340 size). That has to knock it down a notch.
first I found a combo with a .030" over 273, zero decked pistons with a 6cc dome, 57cc combustion chambers, and an .040"x3.70" gasket...and a few other details...yielded 10.7:1 static CR. When I changed the gasket diameter from 3.70" to 4.14", it dropped the static CR (drum roll please)....
0.2:1 from 10.7:1 to 10.5:1...
While there may be a benefit from more appropriately sized head gaskets, I don't think in this case it would produce anything more than a negligible effect.
The reason it's tough to get an actual 10.5:1 CR with a 273 is because there aren't that many pistons to choose from, and the ones which are available off the shelf would require some balancing to get correct and consistent volumes. Another is Mopar left a little to be desired with deck height and shift consistency. Yet another is consistent combustion chamber volumes. Compounding those factors is that every cc added or removed has a greater impact on the outcome compared to if you were dealing with a 340 or 360. Basically, if you want an honest 10.5:1 273, you're going to sink a good bit of money and time into thorough blueprinting and parts selection.