I just calcualted how much time does it need to crack puzzle 66 by bruteforcing
 
The answer is: Infinity OR almost Impossible.

What's insane is, even if someone managed to get 100,000 GPUs, you still would need 3.5 Months to go through all the 66 range. Not to mention electricity cost of the 100,000 GPUs. I think this puzzle is still infeasible. It's worth to mine better than trying to solve this. Or if Satoshi increases the prize again by 10x, so for puzzle 66, 166.6BTC, for puzzle 67, 166.7BTC and so on. THEN it will be worth investing huge mining army to destroy it. Other than that it makes less sense.


What's feasible though is puzzle 125, because of its public key.