Given the options, I would go with a complete engine. You will be better off in the long run. Be sure to pre oil the engine once its in. Do this before initial statrtup and the engine will live much longer. If un-sure what I'm talking about, please say so.
If your engine external components (intake/carb, alternator, water pump, exhaust, PS, etc.) are in good shape another option for you to consider is buying what I call a base engine. You can get an all new base 4.3L GM marine block with cam and crank, heads, oil pan, etc for less than $3,000. I replaced by 4.3L a couple of years ago with a base engine. Originally I was going to rebuild my old engine but after checking the latest machine shop rates to bore the block, recondition the heads, gaskets, etc. decided it was more cost effective to just go with the base engine.
Just another option for you to consider along with all the other options mentioned in the previous responses.