Hey we're on the same path, I also did the same thing recently, 3.0s are a great platform to start with and if you shop around a few good deals on a roller.
I would not worry too much about the drive system for now and hop ups etc, just get going, with the old gear / what you have.
What size of track are you planning to run? This will help decide if a 13.5t or lower is really needed. I think in general you want top speed at the end of the longest straight for example
I run / gear slower for more control and I have to be patient, the first few meets the you'll notice a step improvement and then the gain gets less, here you can start to fettle with set up and start looking at what the top guys in your final are doing in the attempt to get into the next final up etc. Everyone is generous with their time and knowledge. Just go out there and enjoy.
Re Sr drives I have run both one in a 6.5t motor and they are still going. I now have a 17.5t set up as well so you've given my the nudge to sort out which drive is in which car now 😎.
Keep asking / posting the tlr are out of favour at the moment so there are few of us around