My 6.2 sounds like a stonecrusher when it starts below +20 degrees..
IF it starts!...for the first few minutes you wonder if its going to lose a piston or just come to a sudden halt,but so far it hasn't...once it warms up its almost silent compared to a cold start..
So far I've been pretty lucky about getting it started in frigid weather,I only have 7 working glow plugs (maybe less now!),and I only had to boost the batteries or jump start it 3 or 4 times in a dozen years...but we dont get as cold here as Nebraska either...it stays above zero pretty much..
I rarely plug in my block heater,its only an inline heater hose type,I think all it does is make sizzling noises,the hoses only get hot for a foot or so away from it...engine doesn't even know its plugged in!...
I see a factory freeze plug heater is still in the block,but the cord was chopped off and is just hanging there--probably got snipped at the junkyard when they pulled the engine out of a donor...(the engine in my pickup isn't the original)..
The biggest improvement in cold cranking speed I've noticed was using lighter oil than 15W-40...the gear reduction starter helps,but I didn't see a big difference between that one and the old non-reduction one I had in it before really..my oil pressure has dropped a lot at idle when its fully warmed up though,that bothers me some..