So I just had an expansion hose fitting snap on me. How undrivable would this make it? It's the larger hose of the two going in the bottom. 

With it snapped off, it is undriveable. You have gasoline vapors coming into the passenger compartment. You need to plug the hole in the vapor tank and plug the vapor line. This may mean a more difficult time in filling the tank.

Ideally, you would remove the vapor tank and the broken fitting. Plug the vapor lines in the short term and find someone to weld the fitting back in place.

I just noticed a used 280Z tank at ZCarSource for $49, but I suppose they're different for the 240Z.

Looks like quite a few variations on that tank over the years.  Click Engine then Fuel Tank.

http://www.carpartsmanual.com/datsun/Z-1969-1978

I don't have welding or brazing gear so if I had that problem I'd be looking for a mechanical solution.  It would be good to see what's left on the tank.

 

If you don’t have access to a welder/brazer the next best thing is JBWeld…

 

I have some used 240 tanks if you want to go that route. Brazing or JB are also good options. 

Another option is to just get rid of the vapor tank.

http://www.zhome.com/Racing/FuelTankVaporLineMod/FuelTankVaporMod.htm

 

12 hours ago, texasz said:

Another option is to just get rid of the vapor tank.

Maybe you guys don't know this but a euro 240z has no vapor tank to begin with.. When i saw the tank (in the 1990's) the first thing i did was get rid of it!  A euro fill hose has one little pipe and that is connected to a single pipe on the other end top of the tank.. iirc..  see a serv.manual.

4 hours ago, dutchzcarguy said:

Maybe you guys don't know this but a euro 240z has no vapor tank to begin with.. When i saw the tank (in the 1990's) the first thing i did was get rid of it!  A euro fill hose has one little pipe and that is connected to a single pipe on the other end top of the tank.. iirc..  see a serv.manual.

Early Canadian market 240Z's had no vapor tank either, until about 9/70 I believe. They were known as non-emissions models, HLS30UN. Here are a couple of pictures of my 7/70 Canadian car showing the original vent hose routing without a vapor tank. The hose tee's off the tank vent hose going straight up and then loops back down going through a hole in the floor behind the plastic trim panel.

Getting rid of the tank seems to be pretty simple, too bad everything else needs to be plugged off.
The hose that broke off is the one used to T into the filler neck (It's the largest one of the 3).
The hose that vents outside looks to be a larger diameter than the existing line.

I would have to cap off the outlet at the far end of the tank and the one above the fuel level sensor.
The existing hose is in ok condition so I wont have to deal with the 180 bend.
I think I need to get a 3/4" to 5/8" barbed fitting to go into the filler neck. 
5/8" is too small for the filler neck as shown here.
Then I would need a 5/8" T for the existing line and the new line that would go outside.

To plug off the existing lines. I was thinking of just going the easy/non permanent route, and plug the ends of the hoses inside the car.
A threaded brass plug with a clamp is what I was thinking.

Or a brass Pex piping plug from Lowe's or depot

Isn't getting rid of things because they're hard to fix a form of giving up?  The vapor tank actually makes the car more of a pleasure to be around, and removes one "why  do I smell gasoline?" source.  There is a lot on a car that can be removed because it wasn't there in the past.  Go back to carbs, remove the PCV system, get rid of the charcoal canister, go back to points, yada yada yada.

I'll bet that there's enough metal left that you could just find a threaded fitting and "self-tap" some threads in to the tank.  Use some sealant or epoxy on the threads before final assembly.  If it doesn't work, then go ahead and "delete" it.  At least give it a shot.