Another idea is the hpop. Some of the 99s came with a 15 degree hpop. The later 7.3 had a 17, it gives more pressure to fire the injectors. A used 17 is really cheap or consider replacing or modding yours. Another good idea is Valvoline extreme blue 5w40 full syn. I fired my last 7.3 in -21 by cycling the glows twice and it fired and wasn't plugged in.
