basic question about leaf springs--

Ok here's a basic question about leaf springs. When the spring changes shape
the top leaf has to get longer when it flattens out --doesn't it?. The other leafs just slide, when they get flatter, but the top leaf is bolted in. Where does the extra length on the top leaf go?.

thats what the shackle is for

Yeah, the shackle swings on a radius to accomodate the effective change in length (the eye-to-eye distance).

On the XJ, the spring is basically flat at rest, so it's already about as long as it ever gets. It'll get shorter under both droop and compression. For spring-under vehicles with initial arch, though, the spring does get "longer" when compressed.