I use elixir heavy 12 strings 13-56 on my Fender 12 string 25.4 which is good for slide in B but I think you want to look at the longer scales 26+ for picking.
They make baritone strings for the 12 string but I think you might want to consider a second nut cut to accomodate that set up the bass is very heavy as mentioned earlier.
Vintage stella would be my choice for a baritone along the lines of Willie Mctell but I believe he played way down several steps and may have used an octave on the b string to provide tension instead of a matched.
If you're in the UK you might look into the Paul Brett - it's well thought of
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