Super 6 conversion

If you are paying $300+ for the SS setup it is absolutely not worth it. I would go bigger exhaust pipe, HEI and compression. You will see a bigger difference vs the 2bbl alone.
The stock 1bbl won't be a limiting factor until you get into the cam/head and supporting mods.

I had a 67 block with the stock cam, holley 1920,9.5 to 1 comp that ran much better than a 70 Dart with just the Super Six and 2 1/4" exhaust pipe off of the manifold.