The biggest Northerns I have caught (three that went 45" and one that was a good bit larger) have been while trolling lake trout lures down
almost at lake trout depths. They seem to like hitting the lures when I stop trolling and start reeling upwards. At least a couple of these big ones (maybe all of them) hit on 6-inch Sutton Featherlight Spoons using 2 oz clip-on weights during slow trolls. I know I was trolling in water between 45' to 60' depth. The other LT lures I might have used would not go deeper than 15' - 20' deep but both Northerns and LT will come up a considerable distance to nail them if they're active. The really big Northerns seem to hang around the thermocline during the summer months.
In mid-May the story will be different as big Northerns (and LT) are in shallower water. My brother nailed a huge one - it rivalled my 45's - at the surface on a "Fire Tiger" Rapala just below a waterfall at that time of year. In fact, that was the ONLY lure he used during that entire trip. In addition to his huge Northern, he also boated the largest Smallmouth I have ever seen. That was a hot lure that Spring!
I've used some gigantic Daredevils, 8-10" white plastics on huge spinnerbaits, etc., and have had successes when purposely fishing for Northerns but the REALLY big ones I've landed have come while I was fishing for LT. Makes sense, too, since I fish for LT a heckuva lot more than I fish for trophy Northerns.
My old rule of thumb for catching big Northerns (I seldom purposely go after them, anymore) is: take the biggest danged hook you can find, attach a rusty-old Schlitz beer can to it, and troll it at about 15-20' depth about 15-20 yards off of any rocky point. Works like a charm!
Jimbo