Don't you mean 'raise' the compression, KK? The 273 chambers (assuming closed chambers) will be smaller. The head gasket will be thicker; the net ought to be a few tenths increase in compression ratio on the'69 318. But that is assuming closed chambers on the '67.
If the OP's 273 heads are open chambers like the later 273/318 heads, then the thicker head gasket will lower compression about 0.3 points; not a whole point. It is still going to be up at 9:1; better than the later 318's. The OP could go with the .028" thick head gaskets and get back up 9.2-9.3.
But who knows what condition the cylinder bores, pistons, rings, and valves/guides are in and if it will make compression in all cylinders anyway! So the heads and gaskets are kinda a moot point....