Personally...construction.
If working on Mustangs is a hobby, don't make it a business. Keep it enjoyable for you. Also, keep in mind that Mustangs (and performance cars in general) are a hobby for many, so when the economy takes a hit, spending on hobbies tends to go down.
Depending on the field of construction, work may or may not dry out depending on if you are building new houses, or in an in-demand trade like plumbing. I was an electrician for 10 years and through 2008. We slowed down but still had enough work to stay busy and well-paid.