700 x 38c gives your a circumference of 2192 mm
650b x 50c gives you a circumference 2149 mm
https://www.bikecalc.com/wheel_size_math
For what it's worth, Heine likes 650b better when running fat tires:
https://www.renehersecycles.com/how-...should-i-ride/
I don't know if there's a vintage bike out there that can give anything much wider than a 38c. But you can get to your desired width running 650b. But you already know that.