I did a 383 out of a 71 superbee with all factory parts into a 67. Manifolds from the 71 cut the column housing back to the firewall and used two dodge ram mounts they were the same but thicker. Bench seat with the 71 slapstick mounted in front of the seat. had to notch the K for the oil pump. I believe a 73 spool mount k would be a bolt in. SteveSeeing how the 400 did not even come out till 1972 and the Duster never came with a big block, I think it is either a mis-type or somebody is trying to pull somebodies leg.
Ever try to put a BB in a Duster at the factory without BB specific parts??
Until then......