Any other preventive/Needed maintenance i should do on the engine or tranny?
Before you install a 30" lift with gangbanger wheels and tires tall enough to embarrass Bigfoot, before you install a bizillion watt amp driving a boombox left over from a Jimi Hedrix concert, before you install a purple LED bulbs everywhere, before you hang fuzzy dice from the mirror.....
Change all the fluids in the truck. ALL of them--diffs, T-case (if 4wd) trans, power steering fluid, coolant, and of course the motor oil.
If it's a auto trans the best thing you can do for it is get a pan with a drain plug or install a drain plug--keep the fluid bright red. Every third or fourth oil change pull the plug, drain oil, replace oil. Start with a new trans filter then don't worry about it until 30k or so.
Change the fuel filter--immediately. Then change it every 15-20k. If it's a quality fuel pump and hasn't been hurt by not changing the filter every 15k or so it may outlast the rest of the truck.
Flush the cooing system. A
LOT of GMT400's have had conventional coolant mixed in with DexCooll--bad ju-ju.
Grease the chassis.
Electrical components and sensors think, AC Delco or Delphi. Although other companies have upped their game in recent times, I would still stick with the above for cap, rotor, and plug wires. If you can find a really dark spot tonight, light up the truck, let the motor warm up, lift the hood, let your eyes adjust, and check the plug wires for arcing. It might be entertaining.
Most (but not all) hard parts think GM OEM original when possible or affordable.
Odds are your truck needs steering parts, maybe suspension parts. Spend money making the truck "straight" before starting down the slippery slope of mods.