The simple explanation is that 340 Swingers came with dual scoops in 1970...Slant Six and 318 Swingers didn't come with the scoops. Most people who restore '70 Swingers will put that option on themselves, regardless of which engine the car came with from the factory.
To the OP: Beautiful car, it makes me want to paint my '70 Swinger black even more now. Best of luck with the sale!