Vacuum line on petcock goes to #6. If you're desmogged, I don't know why
it would matter which one it went to.
The thing with the T is a vacuum break or something that is supposed to be
there to insure that your tank is venting.
Our tanks are weird (well, probably not to whoever designed them

) ... If you
take your tank off and set it on its nose (it balances well there) it will set up a
vacuum/siphon and a gas-fountain will start shooting out the vent tube. If you
pop open the lid (bad idea while your tank is on its nose if there's much gas
in there) you can hear the vacuum break and the fountain will stop.
-Mike