You would be saving money because you wouldn't have to possibly pull the engine and take it apart after you've already had it in your car. If you want to biuld the block, you're going to want to do it outside the car. Either way, you're spending extra time on labor if you build the engine in two stages, and you're also taking the risk of breaking things again when you make the major mods in the second step. If you want to swap first, it's still a good option- just make sure you know what you want do first and have a solid plan set out for yourself before you start.