I've found one of the best tools to help a locked-up engine start turning to be a carpenter's wrecking bar about 3 feet long. Use the chisel-tapered end in the flywheel teeth at the bottom of the bellhousing after removing the dust cover, and pry against the bellhousing to get the flywheel to turn a tooth or two in one direction, then go to the other side and turn the opposite way. Don't break teeth off the ring gear, but if you're careful, a lot of turning force can be applied that way. If you're not successful in getting it to move, remove the oil pan and the cylinder head, figure out which piston is moving downward by looking at the angle between the connecting rod and the crankshaft, and give that piston a little gentle persuasion with a block of wood and a sledgehammer while someone else keeps the pressure on the flywheel with the pry bar. Pick out another piston that will help the crankshaft move the other way, and repeat the process with the sledge. Using a more effective penetrant than Diesel fuel might be a good idea, maybe PB Blaster or Kroil.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!