not much really. the 250 uses the same bell housing bolt pattern as the 289, so the V8 will bolt up to the transmission, and the rear end should be an 8" already, so no changes there. the transmission is already strong enough for the V8 so again no changes there. the front suspension is the same, including the spindles, though the springs may need to be upgraded to a stiffer set, i recommend the GT springs front and rear.
things that will need to be changed are;
the chassis mounts that the engine mounts bolt to
the radiator outlets depending on what water pump you use
and all the usual bits and pieces like wiring, linkage, fuel lines etc.