My resin casting got smoking hot and cracked, was it too thick?

What you saw is exotherm, the heat given off by the curing reaction. In a thick mass that heat builds up faster than it can escape, so it runs away and cooks the resin, and cracking, yellowing, steaming and a strong smell are the classic signs. General-purpose laminating and coating resins are formulated for thin sections, so a deep single pour is exactly what they struggle with. For chunky castings the usual approach is either a resin designed for thick-section casting, which cures slower and cooler, or building the depth up in thinner layers and letting each one go off before the next. Keeping the mixed material cool to start and working in a cool room also slows things down. Tell us roughly how deep you are casting and in what, and we can point you at the right resin type.