Don't get me wrong here, it's your car & you can modify however you wish, but why hot rod a Sandman ?

To quote your words:- "I've wanted a genuine Sandman for years - ". If so, why not restore it.

You could pay less for a plain jane non-Sandman & fit the Tremec & BBC to that & it would be less work. I can't see why you would do this to a genuine Sandman.

Dr Terry