Yes you will be overweight I had a 2004 LB7 (basically the same as the 2005 LLY specs) 3500 Dually and the max as I remember was 14.5K and it did O.K. but struggled going up the bigger hills and overheated some so went and traded for a 2007 LBZ Dually and they the are rated for 16.3K and have more power and the 6 speed Allison Also do not forget LLY's had a Overheating problem because of a restricted turbo mouthpiece which can be solved pretty easy but anyway yes you can do it but you will be overloaded do yourself a favor and a least step up to a 2006-2007 LBZ Dually they are much better choice for what you are looking at
