Dakota 5 lug wheels came on trucks 1990 and older, 91 and up are all 6 bolt wheels. Can't speak of anything newer the 2004.
90 and older were either 7 1/4 or 8 1/4. The 9 1/4 were all 6 lug that i know of.
A rear wheel drive Dakota has the axle mounted above the springs, but a 4x4 Dakota has the axle under the springs.

If I were in your shoes, and had a chance to put an 8 3/4 in a 68 B body with a slant 6, I would not even consider anything else. Keep in mind, after 70, B body rear ends got wider and may not fit under your 68. Gene