Depends on what you want to use it for. If it is an everyday driver a cheap and easy upgrade would be a 253, trimatic and a taller diff like a 3.08. If it is a car you don't use very often a 253 using your 4spd and a few more bits and pieces like gearbox input shaft, bellhousing etc. Either will bolt straight in with no mods, but you will need a new radiator, throttle cable, exhaust plus hoses, wiring etc either way. As Innuendo says best bet here would be to buy a whole car with the lot in it. This sort of thing, don't be scared of the price as there is money to be made back by selling off the car once you are done:

http://www.ebay.com.au/itm/HQ-Holden...0#ht_70wt_1133
http://www.ebay.com.au/itm/HOLDEN-HZ...#ht_120wt_1133
http://www.ebay.com.au/itm/HZ-Kingsw...#ht_493wt_1133
http://www.ebay.com.au/itm/HQ-HOLDEN...#ht_101wt_1133