a 70g for a Ball python? Way too large IMO, even for an adult. It would definitely be hard to maintain the temps and humidity. That is why I switched to tubs, the tanks were too heavy and hard to clean, and I was unable to get the right temps and humidity down. I would suggest switching to tubs, they hold in humidity very well and it's easy to control temps.

But substrate was all you were asking, you would have to do a lot of misting with certain substrates because heat lamps are nutorious for drying out enclosures. I live in Phoenix, the valley of the sun, so of course my humidity is going to be lower than anything. Which is why I use tubs. I tried cypress mulch with my tank, so I suggest that but I will say you will need to mist it a few times a day. Also look into eco earth, it comes in a block and retains humidity pretty well.