start calling you local junk yards, engine recycling places. part outs.
at least with most junk yards you get a warranty to cover the engine if its bad. not the labor to swap it.
pick up a new gaskets for the oil pan, lim and sc, and valve covers. swap all that before it goes in the car.
your not limited to just a GP's engine, regal gs, rivieras, bonnies all work. you need to swap some parts from your engine to the new one.