440 to single plane or not?

Got a major problem w/ the 440 in my duster. i've got 11in wide tires out back, it will spin them in 3rd gear at 30 mph when you hit it hard. I bought this car a few mths ago and still haven't been able to feel that 1st gear pull. i have to granny it from 1st into second, then in 2nd if i dont ease on it, it will break em' loose. I understand going to a single plane like the m1 will cut bottom end and raise top end a bit. i don't think i'll mind this given my current situation. what do you guys think i'll gain up top? and how much would i be losing down low. heres the specs:
67' 440
915 mildly ported heade
.509 purple cam
750 holley dp
3200k converter
all the related goodies

would i be better off going to a 850 cfm w/ a single plane or keep the lowly 750?