Hello Paul, I have been in the same predicament as you, doing the same thing. I took the old engine round to a tyre fitting place and they used their air impact gun. Same thing as Ian suggested. My 12 point socket suffered badly but the bolt came off.
Just recently to get the crank shaft pulley bolt loosened on an old toyota corolla. I welded a six point socket to a length of 1/4 inch flat bar about 2 inches by 20 inches. I used a socket that was meant for air impact tools. This socket had a very thick wall.
Using blocks of wood the socket and bar was wedged hard against the pulley. As another man advised further up the thread, I also took the spark plugs out. A quick blip of the starter with the bar rotating just over
90 degrees and hitting another block of wood resting on a strong part of the car body. Two goes with the starter motor and the bolt loosened. No damage to the socket or bolt head.I now have three special tools like that now. I wont risk my socket set extentions when lumps of steel and spare sockets for chinese air impact tools are so cheap. Very easy to arc weld a socket on to the steel bar. A professinal welder probably wouldn't charge you as it is so quick and easy if you don't have a friend with a welder.
I am just mentioning the special made up tool just in case you can't get to a tyre place to use an air impact tool. You could fit the starter to the engine that is out of the car. Connect the battery with jumper leads. Since you are dumping the old engine it doesn't really matter which rough method you use it too hold the engine steady while the larger spanner rotates and hits a lump of wood on a wall or floor. A couple of your mates could push the engine crankshaft pully against a wall. with the special spanner and thin piece of wood in between and you blip the starter with the jumper leads.
Regards John Crighton