i'd leave the gearing alone, and just do what needs to be done to the engine to bring it to specification (for stock). my 1.5-ton b-1-fa easily hauls 4 tons in the bed, and with the same high-range gearing i can do 50 comfortably. i don't know that i'd be able to maintain 55 mph on hills with a load on, or especially towing, but i wouldn't be afraid to put the load on and go slower.
i do have a 2-speed rear, which is a real advantage, as it allows for 8 different forward speeds, rather than 4 speeds. mine also has the booster brake system, with the remote midland vacuum cylinder. the brakes are excellent. if you are going to be towing a backhoe, you'll want to make sure your brakes are excellent, too. still operating with the 4 standard drums, no disc brake conversion (and the single-chamber MC).
i've hauled a lot of stuff in my truck since 2007 when i purchased it, including 16' 8x8 bridge panels (3 at once), ice blocks (up to 4 tons), cordwood (1.5 cords), manure (2 tons), and hay (90 bales), maple sap (300 gallons), etc.
use it and enjoy it.