I own a 2006 Silverado 2500HD. It is the extended cab with 6.5 foot bed. It has the 26 gallon tank. I only get about 250 miles before I am down to the quarter-tank level. I would like to convert it to the GM 34 gallon tank. Is this possible? Is there room for a larger tank underneath the vehicle? How hard is it to do? how much does it cost? Anybody have any ideas?