16yrs old, first car advise?!

well - first - :welcome: :thumbsup:. It does look like a solid car and 10,500 isn't out of hand - so you've put another 3,500 into it? - even that isn't too bad. Stop looking at what you didn't get and consentrate on what ya got. Only you can decide what the ultimate direction of the building will be, but putting a big block in where a small block was is a "thing" - not saying don't or you shouldn't - just saying there is more to it than a simple swap. I like small blocks, so I am going to say sell the 383 and trans (which I don't think you can use easily) and find a 360 and build it. JMHO