I don't see why not. I have 258 cu/ft in the sleeper and about 115 in the cab.
With the insulation in the sleeper, and the 'curtains' shut, it stays cool for a while in 100 degree temps. The sun is the real issue, I have to add the last layer of heat barrier stuff and moisture barrier and then it should be fine.
The cab on the other hand suffers from heat soak, which means after driving a while, the heat comes from the dash and floor. This is where the problem is with a small unit and sitting up in the front won't be an option if you want to stay cool.
The other thing that comes to mind is humidity, with the 5500 btu unit, it doesn't pull as much water out as the larger one but again, the evaporator is small (80 sq/in) so changing the evaporator around to have more surface area may help that.