Upgrades to my stock 273 67 Dart GT
Just my 2 cents...
If you stick with stock parts you will be much happier. Easier to assemble and maintain.
The power increase between 2bbl and 4bbl and dual exhaust will be minimal.
The factory 2bbl was 180 HP, the 4 BBL was 235 but that was due to the cam and 10.5 compression. The heads, valves, exhaust manifolds, etc were all the same.
The exhaust system was single but with slightly larger tube.