Unless you have a transmission jack I would highly recommend 2 people minimum pull the trans out so one can hold the trans on the jack (with a piece of wood in between) and the other to work the jack.
Just unbolt the converter from the flex plate and push it back into the transmission as far as it will go. Only when you remove the converter from the transmission do you need to ensure it goes back in properly and engages properly with the pump and stator shaft etc (you should feel 3 distinct thumps or clicks as it slides and engages whilst spinning the converter).
Genuine rear plate is the best option (not actually seen aftermarket plates). See attached PDF, The big thing to note is that the seal and plate are centred around the crank flange (the weight of the seal plate might push it down some). Holden show a special tool to assist with this but you can probably eye ball it (just make sure the gap around the plate/flange is even the whole way around).
I do believe genuine rear seal plates come with little plastic bushes to help centre the plate with the bolts.
Ohh, clean the crank flange and put engine oil on it before pushing the seal/plate on. Without any oil on it the seal will run dry and can risk burning up causing leaks.
