64 random char can not be bruted forced.
1) The attack space is simply to large.
"64 char" doesn't provide a lot of useful info but if we assume it is something like base58 encoding the possible values are
58^64 = 7.3 * 10^112
Even if we are just looking for lower case (a-z) only it is:
26^64 = 3.6 * 10 ^90
In either case is more possible values then the resulting hash (2^256 = 1.16 * 10^77) so there is a higher probability of finding a collision than the original hashed value. 128 bit is considered beyond brute force and we are asininely way beyond that.
2) SHA-256 is sigificantly slower than NTLM hashes.
From your link
SHA-2 is generally slower than SHA-1
This is academic because the attack space is impossible but the throughput on a single 7970 is about 1 billion hashes per second.
http://hashcat.net/oclhashcat-plus/#features-algos1+2)
So 1.6*10^77 possible hashes * 1% chance / 1 billion hashes per second per 7970 / 31.5 billion seconds per year = 5 * 10^67 GPU years.
With one HD 7970 you would have a 1% chance of finding a collision in 5 * 10^67 years.
With 5 * 10^67 HD 7970s you would have a 1% chance of finding a collision in one year.
TL/DR: not possible
