Bradlee, try calling DAP
, they might be able to get you the NAS chassis. However it might take some time. Getting a used 90 frame is also not a bad way to go. Just as long as it's not too rough. But even a bad rear member is a relatively simple fix.
A TD5 frame wouldn't be bad either. It already has the NAS gas tank mounts (if I recall correctly). All you need to do is move the motor mounts. Relatively easy to do prior to getting it galvied.