Felpor has a full kit for the 3.5 SOHCs that is the VC gaskets and the plug tube grommets.

And while you're doing all that, might as well fo the valve guide seals, and all the o-rings and grommets on the fuels rails / injectors.

And while your at that, change all the water line hoses and o-rings.

And of course new plugs and wires...

Big Truck: 00, 3.5, Endeavor, 5-Spd drive line in hand!
Little Truck: 87, 2.6T I/C, MT, LSDs, Tonneau Top
Her Truck: 03, 3.8, 20th Anniv, 65k
Daughter's: 06 Eclipse, Keeping it Mitsu!
FSMs: MitsubishiLinks.com