There is no replacement for displacement.

The only streetability issues I can see with the big block is the 9 mpg cutting your range on one tank down.

IMO-If there is room for a big block, bolt one in.

Unless you plan on autocrossing the car, (which a good friend of mine autocrossed his BBC powered 78 Malibu), the big block is the way to go.

The bonus to the cubes is, you can build a monster running on the cheap pump gas with a pussy cat idle and have loads of torque.

Another positive-Leave your car alone, build the big block while you still enjoy your car, (since it is still running and not in pieces getting a supercharger added). Once ready to install the big motor, pull the small block, sell it, re-coupe some costs, turn key..enjoy stump pulling torque!

BTW- Your SS has the perfect stance. I always hated how they seem to ride nose up/rear down in stock form. It ruined their look. Yours looks perfect. +1 on the Convo-Pros too.