They are big heavy cars but a shit load of fun on a track

Strip out all unnecessary weight as has been suggested, convert to vented discs on the rear if it hasn't already got them... change pads on the front for Mintex 1144 or better, but also change the discs for new - dont go for the fancy grooved vented and cross drilled type, under heavy use they tend to warp, go for solid vented discs....
Dont worry about the drivetrain for now as you need to get used to how the car handles under track conditions, its completely different to when on the road as you have higher cornering speeds for a start....
Change the suspension for stiffer springs and better shocks...
New tyres are always a bonus but get proper make tyres not the cheap type as they will fal off very quickly under heavy loads, especially at the front...
Expect brake fade - a lot - until you can afford to upgrade to either Monaro brakes or Pontiac GTO brakes... they are bigger and have much better stopping power...
A rollcage isnt necessary unless you intend to get to very high speeds or intend racing it, track days are not for racing but to have a laugh and enjoy yourself under a controlled environment....