You can buy used hard tops for them. Most are steel, some are fiberglass. For top fitment see my page here:
4bbl? The 2100 is a VERY good carburetor for this application, a 304 is a pretty small engine so unless you put a rather small carburetor, the Truck Avenger 470 comes to mind quickly, you won't see any real benefit.
I'm not a fan of just changing stuff just because someone suggested it. You haven't even driven this vehicle by the sound of it. A 304 with a 2100 is no slouch. JMO, YMMV, blah, blah, blah.