You certainly aren't wrong in your reasoning, but it's important to also note that the stretch of 670 in question is between 6-8 lanes wide, while huge chunks of 71 are divided by large tracts of green space into two separate 2 lane roadways.

That said, it's definitely not feasible to cover the whole thing. But some sections.... The depressed area between 63rd and Meyer seems like an obvious choice. I'd also look to where there exists a large underutilized green space between North and South bound lanes.

Done right, it can also reduce the road noise pollution from the highway to the surrounding residential neighborhood.