What is the large tube sticking out just below the front carb?

does it connect to the balance bar?

If so, do they have to be connected with a hose?

can they be plugged?

can they be left open?

It is for positive crankcase ventilation. There should be a hose going from that to the PCV valve at the balance tube. 

They are parts 3 & 5 on this page: https://www.carpartsmanual.com/datsun/Z-1969-1978/engine-240z-260z/emission-control-device.

Hose: https://www.thezstore.com/page/TZS/PROD/15-6211

Valve: https://www.rockauto.com/en/catalog/nissan,1972,240z,2.4l+l6,1209169,exhaust+&+emission,positive+crankcase+ventilation+(pcv)+valve,5052

If you plug the PCV, you risk pressurizing your block and finding out how weak your oil seals are.

If you decide to buy the hose from MSA call them and give your CZC.com member ID, 33599 I think and you'll get 10% off.

@siteunseen what is MSA?  I've seen it many times, but never asked.

Motorsport Auto - http://zcarparts.com/

Do most owners plug the PCV valve and put a breather on the crankcase, OR keep it original with the hose?

Datsun Spirit plugs the bottom, then just a breather filter at top.  

46 minutes ago, HappyZ said:

Do most owners plug the PCV valve and put a breather on the crankcase, OR keep it original with the hose?

Most of the cars I have seen keep the hoses.

Keep the hoses!

Thanks everyone...I'll keep the hose

@jfa.series1what header is that on the motor pic you posted above?  My header won't let me install my carb heat shield.

That is the MSA 6-1 unit, no changes required for the heat shield install.