What does the 'Y" shaped cross over tube between the intake manifolds do? How essential is it? What does it do that the large cross over pipe at the rear doesn't do?

I'm inclined to remove it to clean up the lines of the engine compartment by de-cluttering it where ever possible.
I already removed the air injection system and that help A LOT.

 

That tube provides the suction to make the crankcase ventilation system work (PCV). If you fit a catch can and a tube from it to the manifold elsewhere, then you can remove it and plug the spigots with (eg) silicone blanking caps.

 

Thanks.

 

also provides vacuum to the evap canister system.

 

pic of my oil catch can off the rubber knob at front of cam cover, goes under the manifolds vacuum fitting , and joins the big rear tube together!
no valve in system , just a 1/4" hole restrictor in the knob/pig snoot!
keeps a vacuum in the crankcase , to help reduce oil leaks , especially rope seal engines!
BUT engine response is better (i have no idea why) ?

 

Very interesting..
I am going to look into this more. Especially since you say it helps the engine be more responsive.
That is a great looking engine you have. Expertly detailed.
Thanks.