90 degree pipe with a large end and a small end

I can't quite figure out how to create a 90 degree elbow pipe with 14.5mm ID and 16mm OD on one end and a 18.25 ID and 20mm OD on the other.

I started with a 90 degree arc on a 40mm diameter, created the appropriate ends but how do I blend them together following the arc and tapering appropriately?