Box meister,
Dont get the one Mike has, it wont fit the LS1 style front sump, it interferes with the alternator, or the engine mount depending which way you orientate it.up or down, Gregg and I have experienced this first hand.
Improved racing do make an adaptor specifically for the GTO, Holden front sump fitouts, even stating Holden Commodore and Monaro LS1 on thier site,
This is mine fitted, part number IR-EGM-1007
includes a 1/8th NPT ports for pressure and temp.
http://www.improvedracing.com/adapte...-pan-p-82.html
comes in two versions, -8AN and -10AN
regards
"the other Dave"