Solid red bricks shouldn't crack as they heat and cool slowly with the cycle of the burn. I've got a big stove with built in firebrick but I live in a temperate zone so use loose stacked solid redbricks on either size to control the size of the firebox - bricks either side of the firebox in late autumn/early winter, then just one side as it get colder and then none for a couple of weeks in late winter. Been using the same bricks for years without a crack.
This lets me keep the fire in at night throughout the season without overheating the house or ripping through my wood. When you brick the firebox you're not only reducing the space for wood, you're reducing the amount of air the box can hold (and adding thermal mass) - loading a big box with a smaller amount of wood just means the wood has more surface to air contact and burns hotter and faster. End result is your fire doesn't stay in and more of your heat disappears up the chimney.
