Thanks, tcmtech. I have faith in your experience.
The 3-pt will be on the side with the vertical black bars shown in the Lesco picture. Imagine using a pair forks from the side, except it will be the 3-point.
Travel with the tank full will be very limited, if ever. In fact, the tank is unlikely to be more than half full. More typically it will have 60 to 80 gallons. This is mostly for spraying around the house for carpenter bees and weeds and my driveway for grass. For the field, I have a 200 gal Hardi with 24' boom already on a 3-point. It is a tall, narrower tank.
With fresh morning eyes, I looked a little harder and found this site:
https://www.engineersedge.com/beam_bending/beam_bending8.htm
I assumed a 2000# load equally distributed between two lifts, X3 for "bounce" and safety (i.e., 3000#), and values from here:
https://www.electro-tech-online.com/custompdfs/2012/06/HollowStruct.pdf to find that a 4X2X3/16 will give a deflection of 0.25" while a 3X2X3/16 will be 0.5". So, 4X2 it will be.
Just curious, what do you pay on average for hot roll and tubing? My local distributor charged $0.85 to $0.87 per pound for some hot roll shorts, which seemed a little high. But, I haven't bought steel in a long time. Thirty years ago, I was paying $0.35 to $0.40 for full tubing lengths (delivered), and $0.10 for clean drops at the distributor -- I know today the price is higher.
John