"more HP from a caprice 4.3L lt1 engine?
whats the best option? swap rear ends? engines? just a new whip?"
You could try swapping that 4.3 L99 for the 5.7 LT-1
4.3 is NOT LT-1....it looks the same under the hood but its a weak motor in comparison. Once you have the LT-1 if its still not enough for you, try diff gears and have your computer programmed for those gears and other items such as firmer shift and higher octane fuel etc....