Pull all the glowplugs out. Squirt trans fluid into the holes. Not pour, But squirt with an oil can. The idea is some will splash off the piston top and onto the cylinder walls. Let sit for a couple days. While waiting, Replace oil and fuel filters with new. Change the engine oil at this time. I did this but not necessary is to remove all the belts for the accessories except alternator and water pump. This will make sense later. Pull the line coming from the filter housing to the IP off and run some tubing into an empty fuel can and flush out your fuel system. Diesel doesn't go bad like gasoline does. But, It does collect water and grow algae. This is assuming you have an electric pump. If still on the mechanical, Get an electric and delete the mech pump. If you're wondering why change the filter then flush is because what if the old filter is already clogged? This way, You know it isn't and it will be free flowing. Re-connect fuel line to IP. loosen the line caps at each injector with a 5/8" line wrench just enough where they're not tight. After the couple days to a week has passed and you have clean oil in the crank, get a 15/16" shallow socket with a 1/2" drive breaker bar if you have the room and SLOWLY turn the engine over by hand first. With no glowplugs in OR no accessories dragging it down like a vacuum pump or PS pump it should be MUCH easier. If you can't get a breaker bar in there, a pipe wrench with the jaws covered in tape and rags on the crank pulley also work. But, make sure the jaws are fully against the pulley other wise, you'll bend the belt guard some. If she turns over by hand at least 2 revolutions in both directions, put the glowplugs back in and bleed the spider lines down. If it has air, fuel, and some compression, it WILL run. Mine sat from '06 until now with apparently a severely blown headgasket and STILL RAN.
This is assuming everything else is good and she just needs freshened up to get running. I went through this but mine was a "*** DID I JUST DO?!" when I got it home kinda thing.... There is pictures all over showing it back then.
Its alot and not very detailed but its a start. Google and youtube are your best friends really. Its one to read how, Its another to see it done. I'm sure someone will come here and have better/more detailed info but its a start. 7.3's are just like the 6.9 except one has cavatation problems and the other doesn't but both have the same block.