350 mag bad compression

This is a 1997 stingray 220sx with a 350 mercruiser mag, carburated with an alpha drive. This boat will run in the high fifties, and plain in seconds. Last few times out the boat felt very low on power thought it was to many on boat. I checked all aspects of spark and fuel:carb, fuel lines,dist, wires, rotor, coil, plugs. All checked out or were replaced. Started on the r side with a compression test, 1,3,5,& 7, all around 165. L side cyl 2, 165, next cyl 4, zero! Cyl 6&8 165.
I pulled the L head #4 had en exhaust valve with a chunk blown out of it. Sent the head out got it back in one day,complete 375.00 with a FEL-PRO gasket kit (17230).
O.K. heres where I need the help on the rebuild. Do you use any sealer with the Exhaust gasket, Head gasket, Intake manifold gasket? (bead, coating, spray, nothing) I know the intake manifold takes RTV on some sections but I am not sure about the gaskets.
Also are the rockers tourqed (50)ft lbs or do I eliminate the valve lash on the cyl at compression and tighten additional half turn?

Re: 350 mag bad compression

of course rtv goes front and rear of intake gaket, I use gasketcinch on the intake apply to both surfaces and both sides of gasket then let dry, valve lash is to be removed then 1/2 turn like you suggested none on head gaskets unless they are copper then you need the copper spray i think, exhaust you can use rtv but it might burn off hope this helps. good luck post results when done