does anyone know if the R30 skyline diff fits in a 240k. are they the same?

Pretty sure the diff will fit, but it has different shafts. The DR 30 will have CV joints, whereas the 240K one has universals. If you have the complete DR30 diff and axles, you should have enough parts to do the swap. I don't know whether any machining or modification is needed though.

Also, some of the newer diffs are a different length to the older ones. Just check this before committing.

HTH