'72 240Z Gas tank fit a '76 280Z?

Depends...

if the vehicle has a build date before 6/76, the tank will physically fit. After that, they used a different, higher capacity tank with the space-saver spare and it's not really applicable to that design of rear tank holding structure.

But the small inlet will wreak havoc on the fuel pump inlet if you are EFI, as well as having to replumb the vent lines and evap lines to give you a decent "return line" for the EFI pump's flow capacity. Also, there is no sump in the 240 tank, so when going around corners or accelerating with anythin below 1/2 a tank, you run the risk of cavitating the pump momentarily as all the fuel sloshes away from the puny pickup line and the pump sucks air.

Now, if you are going with a carburetted car, it will be fine.

But the question was "will it fit" and to that the answer is, "yes, if the chassis is before 6/76.

The asker of the first question also might be interested in "will it work?", and I've gone ahead and answered that, too, just to be safe...

What do you NEED to do?

For the most part, there is nothing you can do that requires splitting open the tank.
Most problems can be fixed through an acid wash, and internal recoating.

If tubing needs to be replaced, you can cut it out locally at the side of the tank, and replace it the same way, no need to cut the whole tank open.

The only tanks requiring opening is 240/260 tanks to install baffles, sump rings, etc etc etc...

Presonally, I would not use a TORCH on a GASOLINE CONTAINER....

My preference is to fill it with water, and cut with a die grinder and abrasive disc, until I can get a sawzall blad or jigsaw blade into there...

But like I said, there really is no reason to cut one fully open.

Good Luck.