I totally agree with you about playing Obi Wan; Ben had important information to relate and Mifune’s inability to speak english would likely mean he’d be dubbed. But Vader was dubbed anyway, and he certainly could’ve done the lightsaber battles well. Still, how tall was Mifune? David Prowse at least added a formidable presence to the role. If Vader was shorter and not very muscular (at least not bodybuilder class muscle), I think much of the menace of the character would be lost. Mifune likely would’ve brought more grace and snap to the character’s movements but I can’t imagine his Vader casting such an impressive silhouette, especially in ESB.
Good find! 😃