I don’t really know what I’m doing

My Motto:
I don’t know what I’m doing.
But that hasn't stopped me yet.
(Or: I know what I'm doing. I've just never done this before.)

A word of free advice.
Worth what you pay for it.
Nothing "wrong" with a big block A body.
But if this is your first learning project, put a small block in it.
Keep it simple.
It will go plenty fast and a big block swap makes things have to happen that are more complex.
If the 383 is your choice and it needs rebuilding these southern folks are a good choice if you aren't going to do it yourself.
Reasonable and good work. Even darned cheap to ship if you don't want to drive there.

OEM Engines