very carefully
Which plenum-intake or. . . .
It is on the underside of the intake manifold
yes it is
no, it is very easy, it just takes time, plan on aprox. 6 hours so you do not feel rushed.
Not sure what engine you are working on but if you are using a new gasket there is no need for silicone unless the directions included with the new gasket require it.
You have to remove the intake plenum air chamber to get to the 3 plugs against the firewall. It requires to buy intake plenum gaskets and go ahead and replace the PCV hose on the back of the plenum. Need to get from the dealer. Gaskets also from dealer.
P0300 : Multiple cylinder misfire. see related questions below. Look up TSB-09-05-00. The intake plenum gasket may have failed, causing the intake to suck oil from the upper engine into the cylinders. remove throttle body and look down intyo the intake and check for oil. if there is oil, replace the plenum gasket. the TSB has the procedure. This is very specific to the 98-99 durango.
The manifold gasket is the same as the plenum gasket. The manifold gasket is a barrier between the manifold and the engine block.
remove upper intake plenum
No.
First of all you can get to the first 3 plugs very easily. The other three you have to take off the plenum (be sure to replace the old plenum gasket)
White smoke is coolant in the cylinders. Died and won't restart cause the spark plugs are shorted with coolant. If you keep trying to start it without fixing it you could bend a rod! Most likely it is a failed intake manifold gasket or intake plenum gasket or the intake plenum itself. Don't drive the car cause if it is the plenum leaking coolant into the engine it can fill a cylinder with coolant and do major damage when you try to start it. No way to know for sure which of the three it is without pulling the plenum. If you're pulling that you might as well change the intake manifold gasket too. Could be a bad head gasket