Cowl Rust! What is the best way to fix it?

Working on my 68 dart redoing some really bad repairs, I'm sure it was done to the best of someone's ability but know we have replacement sheet metal. I knew I had a little rust on the bottom of the cowl but didn't realise it was this bad. Well of course I did use a small hammer to see how extensive the damage was. What is the best plan of action? Go in from the top? Or try to do it from the bottom? The engine and trans will be pulled leaving only the rear end in the car along with the front suspension. I have a picture for your enjoyment. Sadly I noticed today the passenger side has at least pin holes in it. Future plans include Heat and Air from Classic Air. Thanks for your help. Jeff