It's common for folks to over tighten trans pan gasket bolts, and ruin the cork gasket. I did this job a while back on my F250 (can I say that in a GMH forum?) and there's actually a torque specification of 15 inch pounds for these bolts (1 & 1/4 ft pounds). You can get a small sidchrome torque wrench graduated in inch pounds (as distinct from its larger brother used in engine rebuilding graduated in foot pounds) just for this job...
This is a way to ensure you get the tension right (even) all around the gasket for a good seal without over tightening one spot and squashing the gasket out the side of the pan and voiding the seal.
It's an alloy housing for the pan and very easy to strip the threads if your at all ham fisted and prone to over tightening bolts. The inch pound torque wrench takes all the guess work out of it.
The wrench isn't cheap (at ~ $150 from memory) and it will sit in your shed like mine probably for years before you need to use it again but at least you then have it when next you need it.
I've always preferred to change the fluid from the cooler outlets while idling in drive with wheels chocked & hand brake on.
One clear plastic tube into a drum of new fluid, to be sucked into the trans by the fluid pump, and the other into an empty drum to take all the old fluid. Just idle the vehicle in park until all the old fluid is gone from sump torque converter, lines and cooler...you can see it all dark brown - when it starts to run fresh red and clean into the drum - turn engine off and re - connect pipe fitting to cooler!
Of course if you want to do the pan filter the best time is to then drop the pan (saving your new fluid in a container to add back in afterwards) and replace your filter and refit the pan with a new gasket and torque it up to specs. Then re add your new fluid. This is the BEST way to get the trans really cleaned out and a new lease of life. If you wanted too you can get your old trans fluid analyzed (westrac here in perth) to detect any problems headed your way with the trans due to excessive wear metals.
I did a mod to mine...where i added a large replaceable engine oil filter "in line" between the trans and cooler.
You do have a pan filter admittedly BUT when auto tannys go they often drop the torque converter first and send shards of metal thru the trans and pump and lines - blocking the cooler for eg...and the metal scores all the aluminium valve bodies in the auto trans housing making it in need of a complete rebuild. If you fir a filter in line to your cooler you can filter out all the metals bits before the fluid gets back from the torque converter to the transmission valve bodies - this - saving this major component from damage...and also keeping the trans cooler clean inside. If you do this and a torque converter lets go, then essentially all you need do is drop the trans and replace the torque converter....and your away again...
The difficulty for you in the commodores might be finding room for the filter - on my F 250 I fitted the filter body and filter up front inside the roo bar...and ran the rubber hoses too and from it to the cooler line also up front - simple cheap quick and effective - also adds another liter or so of fluid of your into heavy towing (which i am....).
I also fitted a transmission temp gauge to keep an eye on trans temps when towing as hot trans temps is what kills transmissions when towing.
used transmission fluid analysis results
Trans temps effects on Trans life.
Trans clutch pac wear materials on the pan magnet...
Too many people are scared of having a go at at their auto trans....a LOT of problems can be avoided with the right attention to detail.
Cheers