There are motor mount holes on the DC electric motor and
you can fashion an assembly across the frame from
underneath that will incorporate these threaded holes or
mounts.
You will want to fit all the parts in place and do a test run
before welding anything to assure that it fits.
Do not weld near the electric motor as there is a real
risk of damaging this device.
Commercial motor mounts are available for popular vehicles
too but you can build your own at a fraction of the cost.
Next, you can tackle the sometimes tricky process of Battery
Placement...
Battery Placement
You have basically two options available to you depending on
the vehicle that you are converting.
They can go in the engine compartment, or in the frame
somewhere. If you are lucky enough to convert a truck you
can also place the batteries in the bed.