I'd definitely drain the water separator first. Whenever I had starting issues with the 5.7L diesel in my 1981 Bonneville, WD40 was my go to starting fluid. You can spray it directly into the intake and the engine will idle on it just fine until your injector lines are purged.
Thanks to some insider (one of my GMI roommates was on the Oldsmobile V-6 diesel development team), I got 269,000 miles out of that car before a Texas hailstorm literally destroyed ever horizontal surface on the body. There was nothing quite like getting over 800 miles on a tank of fuel on the highway.
Here's a recent startup of d dormant D7:
https://www.google.com/search?q=1959...4WkJawTks,st:0