i have a 7.3 and it was hard to start..... i found the simplest thing helped fixed it...
if you have good controller good plugs which it sounds like. run a seperate ground wire to the engine from one of the batteries and run at least a 4 gauge wire to the controller relay from the positive post on battery. this gives full potential amps to the glow plugs and they will heat quickly, dont forget part throttle when cranking.... mine starts at 30degrees within 6-8 cranks