The common companies with direct fit kits are Zex and NX. I bought an NX GM EFI kit and bought their throttle position sensor seperately. I also bought an MSD window-switch.
The window-switch is important for us manual cars(yes, it's a good thing for the auto cars, but I'm addressing what I know right now) because the car has a rev-limiter. A rev-limiter will cut the ignition intermittently at a certain RPM to keep the motor from going over a set RPM. When it cuts the ignition if the nitrous and fuel is still pumping into the cylinder it is a very bad thing. If you've ever seen a nitrous"cough" you know what I'm talking about. It's like a backfire that I've seen blow intakes and carbs right off the engine.
The companies have made sure with their suggestions for the jets you don't go lean which could burn a piston. I believe Rod1 has done some dyno-time with data-logging for Vince at Trifecta. Some have run 75hp shot without a tune. I believe Rod1 has gotten all the way up to 125hp shot with the tune and his jet numbers are completely different from what the companies are recommending. The tune is important because the higher shot you get to starts requiring timing to be adjusted and spark plugs to be changed.
Purge kit. Google it and look it up.
Bottle warmer. Also a good thing.
My suggestion is to start low to make sure everything is acting correctly and increase gradually. A couple of the first to try nitrous had an issue with his Zex kit having the wrong fittings. One had an issue with the Zex set-up not working correctly and he actually ran slower(I believe he was adding fuel but not nitrous). I don't know how Zex was when trying to get it all corrected.
There is someone on here who owns or works at Nito-Dave and he has been talking about doing a plate set-up that would install behind the throttle-body like he has done with the LS-motors. I don't know how far he's gotten but I do know their stuff works and they actually race.
Maintaining the kit is the same between the v6 and any other kit.
The added torque is incredible.
I wake up in the morning and I piss excellence.
America is all about speed. Hot, nasty, badass speed. -Eleanor Roosevelt, 1936