Does anyone have a link for info or a step-by-step guide to upgrading from the stock single to double chamber master cylinder on a '66 vehicle w/ wheel drums?

Mopar Action magazine has covered this before. They may have some articles on their website. Look for "Disc-o-tech".

Ebooger has covered it in a separate article in mopar action , it's actually pretty easy to do.

amazing what you can find with that little search box at the top of the page https://board.moparts.org/ubbthreads/showflat.php?Number=5679008

Thanks for the replies - for 4 wheel drum brakes, is the pressure equal at all 4 corners? Or should there be a front/rear bias?

The pressure would be equal with a simple splitter block. you would use smaller rear wheel cyls to adjust the bias to keep from locking up the rears