I have a Larsen 2/70NMO on the way so I need an NMO mount as it comes with the Larsen coax which already has an NMO mount on the end. I took a look at Diamond's website and it's a bit confusing. Either they include the cable, which I don't need - or they have an adapter that changes the NMO mount to PL259 which I don't think will work with the Larsen coax.
I find this confusing...
Typically, the PulseLarsen NMO2/70 antenna would be a whip with a
matching network at the base, that screws into an NMO mount.
Antenna:

NMOTLP trunk lip mount:

That round piece on top of the mount is what the antenna screws into.
The mounts can be several different types: magnetic, through-hole, or,
like this specific one, designed to attach to a trunk lip.
Generally, NMO mounts and antennas from the various manufacturers
are interchangeable (the original design was a "Motorola mount"),
although some Japanese / Chinese versions might have a slightly
different thread or dimensions.
The important point in this context, however, is that the cable is
attached to the mount, not to the antenna.
There's nothing particular about Larsen's coax - the mounts that
I have all use RG-58, which can be adapted to whatever connector
you want. (A solid polyethylene dielectric is better than foamed
in this application, where it may get squeezed in a door.)
So as long as the K400 NMO mount has a cable attached, you would
use it instead of the Larsen mount and cable. If the cable is too
short to reach the radio, you just extend it with another piece in
the usual manner.
I found some web site recently that listed a number of brackets for
specific vehicles, formed to fit under a particular screw and bent to
slither out through an opening. But I can't find it at the moment.
Ah,
here it is, from TheAntennaFarm.com
Note that some of these are just a bracket to hold an NMO through-hole
mount, and others include the mount and coax.