Sounds like you're looking for a little more speed from your ride, so upgrading gears will definitely help.
I definitely agree with upgrading to the 4L80E, or atleast a good rebuild. If you choose the latter option, just run it until it goes out. Why go through the trouble and expense of either, until you need to? Just start price shopping now.
As far your tune up. I have run NGK TR55's for years, without a problem. Wires, cap, and rotor, I would either go with Accel or MSD. I've used both, and no issues. I hate using the crap they have in stock at the autoparts stores. Make sure you go with some bigger plug wires, 8mm or so. Also, don't forget the simple stuff, so make sure you change filter, PCV valve, fuel filter, clean your throttle body, and use a good fuel system cleaner. You could go a step further, and remove the air foil from the TB blade. Very easy mod.
As far as performace suspension, I'm not sure what you'd like to do, stay at the same height, or slight drop. You'll definitely want a good spring and shock combo, but when you're talking about a performance suspension on a truck or SUV, it usually goes one of two ways. You either lift, or lower. I doubt you're going to find something that provides performance, but retains the current ride height. OEM style replacement parts will probably provide SOME performance, if you have a lot of miles, like 200+
A well build 4L60e will be more then enough for a 383....... and simple
I'm not so sure on this one. Weight is one enemy of a transmission, and this is a heavy vehicle. I have seen way too many guys ranting about failed 4L60E's, and those were on mostly stock trucks. I have a buddy who's had his rebuilt like 3-4 times, and he has a 93 Fullsize Blazer with that mad 200 HP. I love them in a car, but they just seem to die left and right in the fullsize trucks and SUVs.