Are the antennas interchangeable between a 240Z and a 260Z?

the way they work, yes, the way they look, maybe.

Will

But is it a direct bolt in without modification?

It depends, if you get one new from Nissan, it comes with the electrical adaptor to drop right in, if you get it out of another car, you probably won't get the connector adaptor, and will have to make up something or modify the wiring.

Will

PS, I have a new one in the box, and given time to hunt it down, can provide you with a picture of the adaptor in question, and the partnumber of the assembly.

Will

The early 240z antenna is shorter than the 280z and is lighter built. The 280 antenna is longer and must be partially disassembled to install. Place them side by side and you will see the differences.

Bob M