Here's why:
* Computer Control: The 1987 G10 van's TBI system is computer-controlled. A carbureted engine doesn't have this, meaning you'd lose all the functions managed by the ECM (Engine Control Module), including ignition timing, fuel delivery, and emissions control. These systems are integrated and vital for the van's operation.
* Wiring Harness: You'd need a completely different wiring harness designed for a carbureted engine. This is a major undertaking, requiring significant electrical knowledge and potentially custom fabrication.
* Fuel System: The TBI system uses a fuel pump and injectors controlled by the computer. A carbureted engine needs a completely different fuel delivery system, including a mechanical fuel pump (often driven off the camshaft), fuel lines, and a carburetor.
* Intake Manifold: The intake manifold on a carbureted 305 is completely different from the TBI manifold.
* Exhaust System: While potentially adaptable, the exhaust system might require modification to fit the new engine.
* Emissions Compliance: Converting from fuel injection to a carburetor will likely violate emissions regulations in many jurisdictions.
In short, while *mechanically* possible with significant modification, the electrical and fuel system changes required make this conversion a very complex and expensive project, arguably more expensive than simply finding a used 305 TBI engine. It's generally not a practical or cost-effective solution unless you have significant mechanical and electrical expertise.
