The floor pans on a "B" van are flat from the firewall to the back doors, but the floor pan under the seat area a D150, is raised up to clear the front of the gas tank. I doubt the "B" van seat pedestals will work at all in the D150. Bucket seats out of a Dakota stand a better chance, but those seat brackets will probably need to be modified, and those seats may be a bit narrow for the full size truck.

The bucket seat Little Reds and other Dodge trucks with buckets (with a buddy seat or a consul between the seats) have the low back buckets (backrest no higher then the bench seat backrest) mounted on a tube frame work that actually bolts in the cab at the same locations as the bench seats. There also be questions about the high back "B" van buckets backrest may lean back too far causing an issue between the driver and steering wheel, and clearing the rear window clearance.