I would rebuild the 289. The 351 isn't that much faster to make it worthwile. You can get plenty of power from a modified 289. If you had a 427 or 460 then maybe it would be worth the extra work. A 302 won't make much difference till you try to sell it and then you will get less for it than a 289.
You will sell it eventually because if you like cars you'll want to move on to something else after a few years. The urge to keep building is stronger than the urge to keep what you got.