Coupla things, first you should be posting in the 7.3 Forums (either General or Motor Problems) to get the best response from the most amount of people...
Next, the Glow Plugs are only 1/2 the system you can have problems with. If the Relay (GPR) isn't working right, and they often don't, your plugs might not even be coming On.
Here's a good link on how to check everything -> Troubleshooting GPR & GP's
For the best Cold Start results (say under 35F) here's what I would follow;
Forget the WTS light, it's not in any way connected to the GP system.
Over 20F, wait a Full min, with the key On, before you crank.
Under 20F, plug the block heater in for 3hrs (get a timer so it comes on before you leave in the morning) and wait 1-1/2 to 2 mins before you crank.
If you're routinely under 20, and especially if it's in the single digits, you will see a big improvement if you use a 5W-40 Synthetic motor oil (that 15W is pretty thick in those temps)
And this all assumes the Batteries/Cables/Starter are all they should be so the starter is getting you full speed engine rpm AFTER the load from running the plugs.
Make sure the GP system is fully functional, follow the guidelines above, you shouldn't have trouble...