Few Questions,

You need to "flash" the converter, so leaving as close to idle as possible is the best way, i'de say your fine with the MTs, if its hooking, you'll get better ETs on them vs. a slick, i'd say 4.11s would have been a better choice, but the 3.73s will be a good N2o gear if you ever spray it, i had issues with my duster not 60fting good as time went on, it hooked well for the most part, but the suspention wasn't holding the car anymore, i would look into that 1st. If the converter is a good one, i don't see 4000 stall being too low, or not enough, what brand is it? A converters quality plays a huge role in its perf., When i was raceing my duster, it had a PTC 11" 3000 stall, it would 60ft. in the low 1.6s, managed a 1.59 once, ETs were 11.teens @ 119/120, but it pulled off a few 11.0s, then i switched to a PTC 9.5" 4400 stall, it then ran in the low 1.5s @ 120 & 10.9s with a 1.51 best, so what i'm saying is, unless your converter is not right, changeing it may not be a fix. Your MPH is there, good for 10.9s easy, i would start with the chassis.