Will a LS1 engine fit into the Pontiac Solstice?

No, an LS1 engine will not fit into a Pontiac Solstice without significant, and likely impractical, modifications. The Solstice's engine bay is simply too small and the chassis isn't designed for the weight and dimensions of the LS1. While some incredibly ambitious engine swaps are possible, this is not one of them. The cost and effort to make it work would vastly outweigh the benefits.