I talked to the people at O&N the other day and they said that the 18 gal tank is doable in the 180. That is the bagage wt limit . The woman I spoke to said they have 337's that she would send with the tank and that they get them approved all the time. The tank is $2800.00 complete. For $3200.00 they will install it and get the FAA approval and do the paperwork- thats $3200.00 for both the tank and the install combined. The only draw back is they are in N.E. Penn. They install the tank in a day and a half.