Couple thoughts
You may want to take a second look at the zinc. Zinc is what they spray on a weld before welding, to keep it from rusting. So, I'm not so sure you are going to get your desired effect. If you want a rusty look, may I suggest a steel powder or shavings and then left outside. If you are looking for more of a green/moldy looking finish, copper powder will do that trick. There are also way to make something look aged with a combination of Vinegar and toilet bowl cleaner..... something like that. Read the article a few years ago from one of the websites. Had pics, looked okay.
Oh yeah, as far as that roof, If you are doing an A sedan, I have found a roof off a Jeep Cherokee (NOT the Grand) works incredibly well. If you cut it really close to the windshield, then flip it back to front, the curvature of the Jeep roof near the windshield matches the curvature of the rear of the A sedan quite nearly perfect, and you get some really cool ribs. Oh yeah, you are going to also want to cut along the inside of the big outside ribs of the Jeep roof. That'll give you 46 inches of roof, you only need 42 1/2. Found this out totally by accident. Been using them ever since.
Just a few more thoughts,
Bruce