547 special Buick GN's were built into GNX's. How many 340 cars out there?
When I had my 87 GN, it was a very fun car to drive, handled well from the factory. 4-speed automatic along with fuel injection & a TURBO! I don't really see a comparison here. Almost 20 years of technology ahead of a 69 340 car.
A GNX would spank a 340 car if you had two good drivers with out a doubt. Power brake the GNX & get the turbo wound up & off it would go.