We had time today to mack a serge tank today here some pics .the under side was rolled on a English wheel to fit the  fender 
This to fit 1/2 a gallon of fuel and two pumps  
2x8 AN 
3x6 AN  
0.90 ali sheet 
let me know what you think  
this is just to show you where you can fit a serge tank in a 240-260