expert help needed (my first post)

If you’re starting from scratch, a big block is not much more expensive than a small block, but since you already have a small block, it would definitely be cheaper to stay with a small block if you’re not going to be making big changes. My Duster was a roller when I got it and I found a decent 440/727 for $500. I had some machine work done to the block and heads but I rebuilt it myself because I like the learning experience.
My advice would be to not always go the cheapest route when it comes to buying parts because you’ll end up replacing them for something nicer later on anyway.
The 440 has a tall deck height so it may interfere with the blower motor and brake booster if you have them. Headers are a tight fit but if you get the right ones they’re pretty easy to deal with. I have Schumacher Tri-Ys.