Push/pulling curved surfaces

I feel I should know how to do this but admit to struggling!

Here is part of a garage structure which is curved on plan. The garage door openings also have arched heads at present. Client now wants them flat topped. So the arched bits need to be infilled.

In a flat wall, this would be a doddle. Draw a line from springing point to springing point one side, push/pull to extrude to opposite face, delete curved lines defining arch. Et voila!

But in a curved wall, not so easy. Or is it?

Curved garage.skp (220.5 KB)

Like this?

I did the same thing @Box did.

Thought one or other of you would come to my rescue! Thanks both. As I suspected, it is easy enough when you know how.