Either silicone or epoxy when cured are impervious to gasoline, motor oil, diesel fuel or transmission fluid. I think if it were me designing the box as you describe, where the thin box was attached to the 3 inch stainless tube, I would make a collar from ABS and solvent bond the collar to the ABS box. The collar would be split and either a large hose clamp or perhaps a collar clamp could mechanically grab the stainless tube, the silicone would be just a sealant.
For the barb fittings I would change the design a little bit also. I would solvent bond a stiffener section to the box, and then drill and tap the holes to accept a tapered pipe thread. Here again, the actual plastic to metal bond would be mechanical, and the silicone would only be a sealant.
Another factor to consider is that ABS might not be a good choice for the chemical resistance. Look at this:
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ABS is listed here as being poor in the chemical resistance department to hydrocarbons.
Another problem could be if the temperature causes some mischief. IIRC crank case oil should run about 200 F. If you look at this:
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you might be running into a problem with the temperature especially if you are running hard and your oil temp is running high.
Perhaps HDPE might be a good choice, but that has a lower melting point, and fabrication becomes tricky because you can't glue the stuff. Vacuum forming might get you something but I think the cheap and easy route would be a metal box.