are you geared to 5.29s or 4.88s atleast?
8mpg is horrible for a 22re. somethings wrong there.
i would swap a 5 speed in it would be a lot more comfortable to drive like uphills. my 4runner on 37s with a 22re with 5.29s gets like 23 to 22mpg if not better sometimes.
if u decide to go with a 4.3/700r4 i would get an adpater plate and try to stick with toy cases just because there is so much support for them plus for duals down the road, although the chevy NPs arnt bad i wouldnt run one.