the ATX can be a little trickier to get the rear valve cover on and off... as the engine and driveline is pushed a few inches further back to the firewall in the engine bay i have heard...and experienced.
you have to carfully work the cover around between the firewall and the cam chain when pulling and installing. it can be very tight. i suppose you could put a piece of wood on the oil pan and jack it up slightly to see it that makes it easier. you also have to be carefull that the gasket does not roll out on the firewall side of the rear valve cover... you need a mirror and light to see what your doing, or go by feel., be sure not to use loads of gasket maker... there will be just as much clumping off inside your engine as outside if you use too much, and you dont want that clogging the oiling system of the engine.
i used a very thin bead in the valve cover to hold and moreover seal the gasket, then put the gasket in the valve cover, and then put a very thin bead on the top of the heads. put a little extra where the gasket makes a turn.
yes the valve cover gasket set can be expensive if you go to some of the major parts stores. i paid $140 canadian from a Canadian tire right near me... i should have ordered from RCM as linked to above for $69 (US...im sure +shipping).