There are any number of c/r calculators out on the web that can save you the effort of building a spreadsheet. Here is one from Summit, click here =>
Compression Ratio Calculator.
If you plug in your engine bore and stroke you can back into cylinder head and dome cc figures to get to a final combustion chamber volume. I think you will find a final combustion chamber volume (TDC Dome in Chamber volume ) of 10.2 cc will very closely approximate a 12.5:1 compression ratio for 427 cubic inches.
If you increase the chamber volume by 10.2 cc's the compression will drop to 11.13:1. I think this might be a stretch. Using twice as thick a head gasket would get you to about 10.7:1 which is still pretty zippy for pump gas with the chamber designs Ford used back then.
The BBM heads have a modern heart shaped chamber shape and I believe they were 75cc in size, if I remember correctly. These would likely not fit your pistons which means you would still need to pull the engine down to buy new pistons. The other challenge is, I believe BBM is out of stock on the as cast heads but might still have some ported heads left — supply chain woes.
In the end you might just be further ahead buy biting the bullet and replacing your current pistons and leaving the heads alone — although the BBM head is a very nice head.