Engine bay welding and prep

[9 Deuce GT]

I've done the searches, but still ahve a few questions that were not covered. I am welding up the holes in the engine bay. I want to do this the right way. I have a question about the use of All-Metal that was used by Mansonozz and Almost stock. Conventional thinking has always said that aluminum and steel react to each other and cause corrosion. Will this happen when applying All-metal to a welded up fender or engine compartment?

Another question, is I live in Western NY. The car is garage kept all year long, and has litteraly seen rain three time since I took ownership 9 1/2 years ago. It gets damp in my garage in the spring when it starts to warm up and then get cold at night. This causes sweat or moisture to start. The previous owner drove it in the winter for a few short months. This has started a small amount of rust in seams. I am going to take care of this properly. I am interest in boxing the frame rails when I work on the engine bay. But, I am afraid that moisture will get trapped in the frame rail due to boxing and eventually rust/rot out the frame rail. Will this be a problem?

 

[ALMOST STOCK]

The reason I used the all-metal was because when I told the auto paint supply store owner what I was doing he said the all-metal was the way to go in the engine bay. So far in 6 +yrs I've seen no indication of the 2 dissimilar metals reacting to one another.

I welded up my engine bay 6 + yrs ago and as of to date I've not seen any indication of rusting (bubbled paint being the 1st indication in that area) yet, not to say it isn't rusting it's just that I haven't seen any indication is all I'm saying.
I per-primed the metal and the inside of the frame rail with a red oxide primer before I welded it up hoping to alleviate any rusting problems, but again not to say it isn't rusting as we speak.