Here's what you need to consider:
* TBI System Limitations: The TBI system isn't ideal for aggressive camshafts. Larger cams often require more air and fuel than the TBI can efficiently deliver, leading to poor drivability, poor fuel economy, and potentially even damage. You might need to upgrade the intake manifold, fuel injectors, and possibly even the ECM (Engine Control Module) to compensate.
* Compression Ratio: The stock compression ratio of the 350 in your truck will influence how well a larger cam will work. Higher compression ratios benefit from more aggressive cams, while lower ratios might not see significant gains and could even run into problems.
* Valve Springs: A larger camshaft will require more aggressive valve springs to properly control the valves at higher RPMs. Stock springs will likely not be sufficient and failure could result in catastrophic engine damage.
* Tuning: You'll almost certainly need to have the engine's computer (ECM) tuned to match the new camshaft. This is crucial for proper air/fuel mixture and ignition timing. Without proper tuning, the engine could run poorly, be unreliable, and potentially be damaged.
* Exhaust System: A restrictive exhaust system will severely hamper the performance gains from a larger camshaft. You may need to upgrade to a less restrictive system.
In short, while technically possible, installing a significantly larger camshaft in your 1990 GMC Sierra 350 TBI is a complex undertaking that requires more than just swapping parts. It's a project best left to someone experienced in engine building and tuning, or potentially a professional engine shop. A less aggressive camshaft upgrade might be more feasible and less problematic. Consider your goals and the resources available before making this modification.
