I was able to fit a 28gal tank behind the gaucho. I used 1 1/2" vent & fill connections at both ends of the tank. Haven't decided yet which will be the inlet and which will be the vent. I will reuse the original inlet, and on the opposite side there was a police radio antenna which I will convert into a vent.
Be cautious if you run the vent through the floor. Besides the problem with spiders, you need to avoid creating a siphon.
In addition to the pump outlet, I added a separate 1/2" drain in the bottom of the tank so I can drain it completely dry in the winter. I also pitched the tank 3/4" toward the outlet end.