273 commando Build

Just so you know;
The Wallace calculator predicts, with cam of 260/260/106, in at 104, with a true Ica of 54*, points to 150psi at ~8.85Scr.
But, because it's a solid lifter, I have no idea when the intake actually closes.
Probably a couple of degrees earlier
At 52*, 150psi would be ~8.75Scr.

So; if your Scr was a true 10:1, and
the intake actually closed at 52*,
then the Pressure could be as high as ~179psi;
Or if the Leakdown was 16%, then yur back to 150psi. But if your LD was that bad, she'd likely be a bit of an oil-burner, leading to detonation.
If your pistons are at zero deck, and with closed chambers, your Quench equals the gasket thickness....... your gonna need a dash of luck running @.028Q, @179psi, @WOT, on pumpgas, with iron heads.
So;
IMO, you need to get to the bottom of your actual Scr and true-pressure;
or your problems may continue; asin ............
maybe the gauge you used was wrong.

For a given Scr, your 273 total chamber volume would need to be, see below;
with predicted pressure following, using an Ica of 52*
For 10/1, 62.1cc; 179psi
For 9:1, 69.9cc; 156psi
For 8:1, 79.9cc; 133psi
The Wallace is usually pretty close.

The thing that peaked my interest was;
new rings on old pistons, in high-mileage bores.
How sure are you that;
the bores were round and not tapered?
that the ring-lands were sealing?
the pistons aren't rattling up and down because of too much clearance?
You know that the rings have to also seal on the lands right......... and you admitted that the grooves were heavily carboned..........
If cast rings have to follow tapered bores, the lands will not last long, and if simultaneously the rings are rubbing on carbon, even less long.
If the bores are not round, the oil-rings will leave oil behind. Oil in the air-fuel charge leads to detonation. which creates tremendous cylinder pressure.
And honestly, why would you push a 222*@050 cam, that peaks at under 4800 in your 273, and especially in a used-piston 273, past say 5300 which would already be high for those factory heads (and a 4speed)? Why?
Don't answer, I know why; but it's not working very well, is it?
Just saying.

but what's even more intriguing to me,
is that yur gonna try again
with another 273.
And no, I'm not a 273 hater; yur the one killing them.