Which years came  with propeller shafts with replaceable u-joints?

None of the 280Z's have replaceable propeller shaft joints.  The last model to have them was the 260Z.  And that was probably only the small bumper 260's.

So will a 260 or 240 driveshaft fit a 280?

They will.  But the first year and a half, about, of 240Z's used a shorter shaft.

I found a guy with a 71 240z driveahaft, its 1" shorter. Are the auto vs manuals versions the same?

They are.

But that's probably a very early 240Z shaft used on the cars with the forward-set differentials.  Those used a shorter propeller shaft.  People use them with the 71C transmission conversion, but they're too short for a stock 280Z coupe with a 71B transmission.

Edit - actually I think that it would work but you'd lose that inch of engagement with the transmission.  I think it might be closer to 1 1/2" if I recall right.

Edited October 8, 20214 yr by Zed Head

MSA has replacement shafts. Start fresh and the shafts they sell are stronger and have replacement joints.

Would like to buy used and keep cost down. Spending $20k on a  280z thats would be worth $10k would be foolish.

Do the autos and manuals have different spines on the yolk or different lengths?

CarPartsManual.com

There you will find everything you need to know on model year/configuration. Once again please take a look. It’s the Z Bible on part number sourcing.

According to this they are the same. Everything i have seen on forums, the people say different. As far as the length, they all look about 30".

https://zcardepot.com/products/transmission-driveshaft-slip-yoke-input-240z-260z-280z