i have had both...but right now i have a 2500HD Chevy...obviosly gas mileage will be the biggest difference...i am getting about 12 city and 14highway with my 6.0 with 4.10's...the reports i have read elsewhere have the 8.1 almost in the same ballpark...if you are going to pull, get the 8.1...
i would think that if the ford crew cab would not fit in your garage, the gmc/chevy crew cab probably will also not fit...why not just get a ext cab lariat PSD?
for more info on the gm trucks both gas and diesel, check out
www.thedieselpage.com
hope that was of some help, good luck
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99 F250 black CC SWB lariat 4x4, 4.10LS National Spring 5 inch lift 35's 2k1 chevy crew cab LT 4x4 SWB pics of both at
www.picturetrail.com/4x4