Some questions on the 273 head gasket "thing".

What is your current cylinder pressure?

Personally, on a hi-compression, 273Commando, I think the headgasket-deal is very important.
Going from the ~3.4cc factory head gasket, to the 8.6cc Felpro,
will drop the Scr from a true 10.5 , down to 9.7, which is, as you can see, almost a full point. This doesn't look like much when you see it in this format. But when expressed as a change in cylinder pressure, you are looking at maybe [email protected] Scr, going to maybe 160psi @9.7; so that's like 15psi which is a loss of ~8.6% ; That's big.
The question is;
Can the 10.5 engine even run on pumpgas, without detonation..
The typical upper limit for best pump gas is 165psi; a little more for closed chamber heads, and/or a late closing intake valve.
Therefore;
in this case, 9.7Scr may actually be a good thing over 10.5.

What's not good, is big-bore gasket on the small-bore block, which pulls the fire-ring back about a quarter inch from the edge of the bore. That can't be good. The exposed top of the bore, with it's sharp edge, is now primed to cause detonation under the right circumstances. So, you gotta fix that....... but when you do, that's gonna drop the Scr a lil more. and no matter what, the crevice is gonna be bad for emissions, if that matters to you.

Sydney Australia, is practically in the ocean, so pressure is gonna be iffy at 10.5 anyway.
IMO; 9.7 may even be too much. Changing your cam-timing may help.
Small chamber ALLOY heads will help a lot. You'll probably even be able to run the lowest octane gas with those.
Just so you know;
My alloy-headed 367SBM combo, has run up to 195psi on 87E10 Canadian gas, which is; (R+M)/2 With the current slightly bigger cam, she is down around 187psi....... still on 87E10.

IMHO
for your application, if the pressure is too high, alloy heads may provide the anti-detonation answer you need. Or, you could just mill the tops down on your pop-up pistons.
It all depends on what your current cylinder pressure is, and
what it might jump up to, after freshening.