SEMA makes a product called RustMort that works really well to prep metal. Turns the red rust into a block oxide layer that is then painted over. Ihave prepped metal that I waited months to paint and it was still great.
I must say I have welded on my car with gas in the tank and didn't really think much of it. I do know for a fact that my tank is in excellent shape with no leaks and a tight fitting gas cap.
For gasoline to explode or even burn, the gas mixture has to between lower and upper flammability ratios, too rich or too lean and nothing will happen. Sometimes a full tank of gas is actually safer than an empty, almost empty one.
Because of the multiple contours, it might be easiest to cut a patch out of a firewall on another 67-76 A-body. Some of the patchwork I did on my 71 Barracuda needed to be done in several steps.
I used touch up gun and it doesn't have a large spray pattern and can give you a tiger stripe look. I don't think it would look good for an entire hood. Changing directions would help though.
like previously said, any phosphoric acid solution will work. This stuff goes on very thin and a gallon will do an entire rusty car, almost. I have used this product as well...
I had so much fun building up my 71 that I really have not missed my 383-s alot, although it would have been nice to keep both of them. My very first Mopar and car was a 71 Cuda, so I have always been partial to them. Rod
Although I have a 69 Barracuda dealer brochure that shows a 383-s that has the stripe and the 383-s fender emblem. Maybe just a promo car, one of one, don't know. It also shows standard barracudas with no fish emblem either, so who knows.
Rod