1981 Chevy Truck Identify an engine

Engine Mechanical problem
1981 Chevy Truck V8 Two Wheel Drive Automatic

I recently bought a 1981 Chevy C10 for a restoration project that had the 305 replaced by a 350 from the previous owner. Where on the engine can I find the serial number to identify exactly what kind of 350 it is? I've looked everywhere, but no luck.

Eventually, I would love to upgrade it to TBI or other modern improvement to increase gas mileage, performance and reliability if I knew what engine I was starting with.

Thanks!