The cab-n-chassis duallies were 14bFF's, to the best of my knowledge. It's an odd axle -- it's a regular SRW axle, but with DRW hubs and the brake flanges scooted in to fit the deep drums.
ANYWAY, a D70 from a dually will (1) be wiiiiiiiiiiiiiiiiide, and (2) will have the 1-ton spring perches, i.e. narrower, so you'd hafta cut and reweld them to fit onto a K5.
Unless you want a BIG project, find a 14bff from a 3/4 ton truck or Burb, which will drop in a lot easier.
-- A