I will probably be your most help right now, as we are doing the same thing for a customer currently, including using the same TCI 6x. Sorry if you wanted to be the first

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Most people that make parts for doing the swaps will tell you that gen 3 is the best, as the gen 4 will not only cost you more, but you are going to have to get in bed with someone that is extremely good at tuning those cars to even have a chance of running it standalone.
The Gen 3 does NOT work with TCI's adapter plate. Trust me.
You will need to talk to Coan Racing, they are the ones that make an adapter with a crank trigger.
As far as harness, talk to Hotwire Auto.
Truck or Viper, does not matter as far as putting an auto behind it, but you may end up trying to find a truck pan to modify depending on what you are doing for a front suspension crossmember.
Someone else may want to chime in, but the Gen 2 may be alot easier to do?