- Would holding back the last 6 or so characters be enough to secure the Private Key?
58^6 = 38,068,692,544
I'd say no.
It is even worse than that. If the private key is in WIF form, the last 32 bits are a check code. So, divide your answer by 2^32 and you get about 10 possible keys that need to be checked
The quirks of the encoding make key recovery a bit harder than that, but it is still WAY less than the 38 billion quoted.