I probably have a different opinion than many, but I don't see any reason to press to that much force (of course Andy, awakephd would disagree, as he pressed his cantal to over 900 lbs of press force). Modern commercial cheddar makers don't even do a traditional cheddaring, at least some don't. I would say if 3/8 cut shrinks to the size of rice you are definately stirring to hard. That may be what you are looking for in a cooked curd swiiss or a grana, but not a cheddar. The cheese overall looks pretty good, I wouldn't call that a crack really, it looks pretty well knitted to me. You would dry the cheese to develop a natural rind before bandaging, and if that is what you are going for then you are going the farmhouse cheddar route and they don't get pressed as hard as an american style chedder. A backer block above the follower would distribute the force better and help prevent the deformation. I think your experince when you make a soft bloomy will give you a better feel for how a curd knits and naturally expells whey. I would personally look at the press as a forming tool to shape the cheese more than a whey expullsion device, although it certainly does that. Have you watched any of the videos from Gavin Weber on You tube, he makes several cheddared style cheeses and never approaches the kind of weight you are talking about, 50 lbs is about max in his makes.
have a cheese for your efforts
Take the above with a grain of salt as I am certainly no expert on cheddar style cheese or hard pressing, at least not yet.