I could be wrong but I had a few B200 and B300 vans and they were all 727's with a 8 3/4 rear or dana 60. The rears will fit in a "B" body. Just a 1/2 inch wider, I believe. Many years ago. The dana's were 8 lugs, so could not be used easily.
Yeah. B 200 and 300 means 70s.back then I believe all the vans had 727s.
My b250 was an 87. 3/4 ton 5 lug 9-1/4 rear. Window van. 904.
My dad had an 86 b250 conversion at the same time and his was 8-1/4 rear. Also 904 ( or 998/999 maybe)
Around that time I was seeing a lot of b vans with 8-1/4s.