Re: New or rebuild
It's difficult to respond to your post without a little more information, but generally, the 460 would be the way to go for more power and torque.
Either option is viable, but here are just a few of many things to consider.
How much experience do you have rebuilding and swapping engines?
What do you have for tools and shop area?
How much do you plan to spend?
A 460 swap would (probably) require another (C-6) transmission if you now have the cast iron FX trans. The swap would also require some fabrication and junkyard searching for hard to find (old) parts, not to mention front suspension mods to handle the increased weight.