As mentioned, you will not find plants that are small enough to fit in a 75 gal that are stout enough to support the weight of your Boa (even when she's small)...
Thus you will want to use tree braches or something similar as 'hardscaping' that she will be able to climb on... then you can accent with either silk plants, ferns, climbing plants, etc, etc...
It is exceptionally unlikely that your Boa will ingest any plant kept in the tank, so most plants will be safe for you to use. My suggestion is to go to your local stores to see what is available, decide a few that you like, then come home and research them to see if they are in deed safe...