The biggest killer to the motor is clogged block drains, You must remove the quick connects from the fitting to insure complete draining of the block.
From what you describe you never drained the motor only the manifolds and hoses.
To be safe drain everything ,pull the block drains and probe...
Remove the outdrive and make,beg,borrow a T bar from pipe and a used input shaft welded up. Remove the heads to determine the damage.
spray a -penetrating oil in the cylinders .
Heat the tops of the pistons with a torch `use penetrating so;vent.
add a length of pipe to the T for leverage and try...
