Cool truck! That sounds like a pretty rare option, with the wooden floored-stepside bed diesel.
I think you're on the right track with removing the glow plugs and trying to spin it over first with a few shots of oil in the cylinders. Make sure you spin it over several times and that all the oil or other fluids have been purged from the cylinders before reinstalling the glow plugs. These engines are high-compression with little clearance between the pistons and heads. A little bit of fluid in the cylinder can cause a hydro-lock situation pretty quickly.
Dropping the fuel tank and cleaning it out would be a good idea as suggested. Change the two fuel filters as well. You have a mechanical fuel lift pump, so once the tank is cleaned and the filters are replaced, the easiest way to prime the fuel system is to spin the engine over with the glow plugs removed and your foot on the fuel pedal until you start getting fuel spray coming out of the holes.
When attempting to start the engine, try plugging in the block heater a few hours before to help aid in starting. Make sure the radiator and block are full of coolant first.
Casey
1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter