The 400 frame is at least as stiff as the OE 1135 frame - possibly more so. Some judicious bracing will help.
Any extra grip from modern rubber is moot as these things become rear wheel steerers in the hands of a decent rider anyway - which "proper race bikes" of the era wouldn't readily do.
They have enough surplus HP to carry sideways action for as far as the rider wishes...controllably.
I love 'em.
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