Does a Trailblazer SS / NNBS intake manifold bolt directly onto an older 5.3, what mods are needed, and what are the real gains?

Gains: in Richard Holdener's back-to-back test on a cammed 6.0 LQ4 (cathedral-port heads), the TBSS intake with a 90mm throttle body beat the Gen III truck intake by about 12–13 hp, with two different cams:

6.0 LQ4 test (Holdener)	Truck intake	TBSS intake + 90mm TB	Gain
Low Buck Truck cam	439 hp / 453 lb-ft	452 hp / 457 lb-ft	+13 hp / +4 lb-ft
Truck Plus cam	453 hp / 448 lb-ft	465 hp / 457 lb-ft	+12 hp / +9 lb-ft

Among factory cathedral-port intakes, back-to-back testing puts the TBSS (TrailBlazer SS) intake on top, but it's tall and often won't fit low-hood cars — which is where the LS6 intake comes in. The truck, LS2, LS6 and TBSS intakes all work well; the original LS1 intake is the weak one. An LS3 (rectangle-port) intake on cathedral heads with adapters is a poor choice.

Fitment: the TBSS is a cathedral-port intake like the Gen III truck intake, but it uses a larger throttle body (the test used a 90mm unit; a 3-to-4-bolt reducer lets you run a truck throttle body). It's also taller, so check hood clearance. We don't have a source detailing the rest of the install changes (sensors, fuel rails, throttle control) for an older truck, so confirm those for your year.