[franken]

I'd probably be looking for the spots that leak. The SBC is/was famous for leaks. Clean it and start looking. No fun.

Leaks from the top look the same as ones from the bottom due to gravity so start w/ the valve covers..

Valve covers and gaskets. The sheetmetal covers get deformed and don''t seal so check w/ a straight-edge and beat them into submission. There are supposed to be load spreading things on each bolt.
The in the timing cover seal digs a groove in the damper, ruins the seal, and leaks. You can get a sleeve for cheap.
The oil seals leak at the front and rear of the crank.
Intake swaps often create leaks at the front and rear.

Pulling the engine and doing anything is going to be a multi-day job.