I came from a Hardbody as well, as the new bolt pattern really pissed me off when I first bought my truck. However I am over it now and it doesn't bother me anymore. I could see if you were shopping for used or new aftermarket wheels that can be an issue and limit your selection. Or if you had a bunch of wheel from your HB that you wanted to use on the D40. However the 6x4.5" pattern is not that rare. Dodges use it for their mid size trucks and SUVs. Dakotas and Durangns.
As for the knuckles. Yes they are swappable. I think the main issue is the hub and CV axle interface. I think the splines and ends are different. You would have to also swap in a m205 diff. Brakes are different too (?).
As for the knuckles. Yes they are swappable. I think the main issue is the hub and CV axle interface. I think the splines and ends are different. You would have to also swap in a m205 diff. Brakes are different too (?).