Andy - It looks like you're making an air duct adapter to bridge between a 4-sided opening and an 8-sided opening.

As Michael indicated, selecting the edges on the 2 openings and Blend is the easiest solution.

You note that you want a flat bottom. For that reason I would use a Network.

Draw 4 curves to connect your 2 openings. In the Right View, note the 2 bottom curves highlighted in yellow - I drew those flat.

Select the 2 curves that make up the 2 openings, the 4 curves you drew to connect them, and Network.

Ed Ferguson