got a pic of that light?
i use this one...its a self contained mh/hps switchable from sunleaves...pulsar two-way mini 250
it resides in a small dresser that is 15"x23"x45"tall. its super stealth but i have no pics yet.
with the light in there i have about 37" of headroom.
right now im using it to prepare for the summer season (gotta take advantage of that free solar energy) but the first time i used it i scrogged 1 nlxmix1 in it and harvested 73.2 grams in soil. but i did fight temp problems until it got really cold outside.

just the inline duct fan is not gonna take care of all that hot air coming off your ballast. you'll need to spend some scratch on a fan that can move some serious air

if your capable of doing some modifications i bet you could turn that fixture you have into a cool tube, i would open it up and find the leads that go from the fixture that the bulb screws into. you might be able to solder a longer piece of wire onto it and then you would be able to just have the bulb in your cab in a cool tube