You've got a 275 G tank. Your FB needs to be ~105%, so maybe 30 x 28 x 26? That puts your throat at 175 sq in. Using my throat calcs, choose your shape, either half-moon or trapezoid. Going with a half-moon for instance, input 27 as your tank diameter, 175 throat area, and 27 wide for FB. That'll give you a cut out of ~9 1/2" high following the shape of the bottom of the tank. Your FB needs the same cut-out. Your BP will be above that opening and go across the tank leaving the same 175 sq in on the opposite side.
If you post a sketch of what your planning it would help.