Marine 273 build

I think you'll find the pistons are down further in the bore than you think, and even the 302s will have larger than advertised chambers, and any head gaskets will have a 4.10 bore and be billed for 273 to 360s... It's a struggle.
For clarification - milling does not affect geometry on a Mopar with shaft rockers. I'm going to suggest something different tho. Get aset of Enginequest Magnums and use them. The 5 bolt valve cover will work, an AMC and most mopar lifters are configured for pushrod oiling, you can get cheap rocker gear and valve springs, and they will outflow the 302s in stock form (Magnum based). You may still need to mill them to get the compression ratio up and run the MP .028 gasket.