Quick recap from latest modifications.
After inspection I noticed crank rear seal is leaking. Only way to replace that was to lift out the engine and gearbox, because it is impossible to remove gearbox from engine in the car. This offered a great oportunity for some tuning. I had a 6.0 liter LQ4 engine in the garage, a set of 243 cylinder heads and a 227/234 cam. Heads needed valve lapping and 0.25mm surfacing due some dents. With these heads the stock LQ4 compression ratio is changed from 9.5 to about 10.2. Also I put GTO LS1 accessories to this engine.

5.3 engine with truck accessories on the floor and 6.0 with GTO accessories on the engine stand. GTO accessories made the upper water hose much easier and smoother. Also dropping the alternator down gave hood clearance and made engine bay look a bit cleaner.

"while I was at it" I changed the gearbox from GS5-39DZ to GS6-37DZ. Old gearbox whined a lot and also I wanted that 6th gear which is overdrive.

Had 26x9.5 slicks in the garage, so they had to be tested

I don´t know though if the stock rear end would be strong enough for those...