The GVWR for a 1996 dually is not nearly as high as the 2005 and later F-350 dually's. Even the 2005 SRW F-350 was 11,500 GVWR with about a 4,000 pound cargo rating. I would be recommending a 2011 and later 6.7L diesel dually.
Sure you can look for a good used Volvo, and remove the CENTER axle, then you will have a 450 HP towing beast, and no need to worry about it for another 100,000 miles. Heck the oil changes are recommended every 50,000 miles or 6 months. I know a trucker who got a new rig in October, and had a 15,000 mile oil change just after Christmas, even though the first oil change was not recommended until 50,000 miles. He thinks nothing of driving LA to Portland OR in 2 days.
Any Volvo in the 350,000 to 400,000 mile range would be a great candidate for your use. Selling the center axle can pay for part of the conversion costs too. The rear axle does not have the locking diff, and is not set up for a output shaft, that is why you sell the center axle. You should not be using the center axle as a drive axle.
Normally they come with a pair of 150 gallon diesel tanks, larger sleeper, and lots of space inside! The turning radius is pretty tight, with the tires turning very sharp when desired. It should handle better than a pickup when empty, due to the air ride, and get pretty decent mileage. Check commercial truck trader.com
Good luck!
Fred.