That's a really nice looking mold you have there. Your drying process sounds great.
If anything cracks I'd consider drying it more slowly, but 300 degrees sounds pretty good. I got nervous when I saw what looked like a flame thrower aimed at your wet core!
If one side dries faster than the other side, you get more cracking. The more evenly the
water leaves, the less likely cracks will form. You see, clay shrinks as it dries. Having non-shrinking things in the mix, like perlite or grog, will help, but there's still going to be a little bit of shrinkage with any ceramic material.
I'm thinking that is made with clay and not cement?? Maybe bags of porcelain or stoneware mix?