I just picked up a jetboat project (79 checkmate jetmate) powered with a 460. on mine there are 2 drain caps on the block under the exhaust manifolds a bit more toward the rear on both sides that are a PITA to get to. due to the state of the project I am in, I did not drain my block as it is in a heated building as I am working on it this winter here in MI.
I hope this helps you find what you are looking for.
Now what I wonder is if I can replace these pipe caps blocking off the drains with 2 small ball valves to ease this process come fall and spring?