What is an optimal fuel tank on 1999 chevy suburban?

[rusterline]

Ok. so i have a 1999 chevy suburban 2500 with a 7.4L and a 454 engine.... fuel injected.... and i am kelly blue booking it now and the difference between an optimal fuel tank and with out is $4,000 so i need to know... please tell me how i could know so i know the real price! thanks
OH YES!!!!! thank u i read it wrong... it is optional fuel tank... it has a 41 gallon fuel tank... is that it????

 

[cam#7]

That's a big as they come on this model.