eta: different tank after all.. my apologies..
here's the thread I started about the tank I have..
50 Gallon Suburban Fuel Tank
may be some useful info there for the op regarding capacity and fuel sender..
for reference, there's 231 cuin in a gallon.. measure all sides of your tank like I illustrated in my thread to determine total capacity..
for the angled portions, square off a triangle and use this formula;
volume of triangular prism
sorry for the intrusion, hope something there helps, good luck!