Driveline Angles

[eric8]

I've read several articles detailing LS swaps in our truck that talk of putting a protractor on the new tranny to ensure the 3 degree downward transmission slant. Most of these swaps are using the swap plates that keep the engine in the same placement as the OE sbc engine. I'm wondering, if this is the case, what is the need for checking that angle? The engine shouldn't have moved right? And I've seen none of them mention checking the angle and it being off after the swap. If this were the case, how are they fixing it?