Following up our conversation, the front axle on many medium duty 2WD trucks looks something like this:
http://www.meritor.com/products/axles/truck_frontnondrive/default.aspx
the "drop" from the centerline of the hub to the lowest portion of the beam varies with truck size, suspension design and application. I have seen some with 5-6" of ground clearance on 30-32" tires, which makes it very easy to snag with mild off roading. Lifting the cab or body with springs, bushings, shackles, etc., does not help you here, the beam stays the same. The only way to gain clearance under the beam would be larger diameter tires. The beams are cast, so they are very heavy duty and will be fine if you are traveling state parks, forest roads, into a mountain bike or ATV park, etc. I would not keep a low beam axle for anything more serious than that.
For custom wheels:
Longs Wheels in Jacksonville, FL - call CJ @ 800-874-0869, fab time is about 3-4 weeks usually.
Vision Wheels has a SRW 19.5 (type 81 wheel) in 8 on 6.5, which is a very common truck pattern
Ag Wheel and Tire in Wichita has made custom wheels for a "deuce and a half" and may be able to do others rated for heavier truck use
Marsh Racing Wheels and Allied Wheels - not sure if they have something heavy duty enough, just have to check
You need a properly rated wheel and preferably a forged wheel, which is stronger than a cast wheel in almost every case.
Mitsubishi FE84/FG bolt pattern is 6 holes, 222.25mm bolt circle diameter, 127mm offset (not backspace), and wheel at mounting surface is 9mm thick.
FE84 Crew Cab Fuso Front Spring Size is 1200mm length (not sure if this is spring length or spring eye to spring eye), 70mm wide, two leaves are 10mm thick and three leaves are 11mm thick.
Sprung weight (does not include axles and wheels, etc, basically what are the springs holding up in the air: Front (3700 pounds) and Rear (900 pounds)
FE84 Crew Cab drawings show about 7" clearance under the front beam axle at full axle load.
Stock tires are only about 30" tall, so you will need to figure your lift based on how much larger of a tire you want to run, how much articulation you think you will see, etc.
Center of spring to center of spring on FE84 front is 31.77". A standard medium duty truck frame is 34" wide. FE84 frame is 29.65" wide (outside edges)
For springs check Deaver and Alcan, they will ask for quite a few measurements (width, arch, bolt size, etc) from your current springs and will then make a new spring pack to your spec. Thinner springs and more of them to make up the pack will ride better than fewer heavier springs with the same overall pack thickness. You could also look at a lightweight spring pack and a air bag helper system (large bags/higher volume ride better than smaller bags) so now you are not relying on a stiff/heavy spring to take the load, which gives you a harsh ride.
Good shocks also make a huge difference, the factory ones are set up for a work truck, not for the comfort of the driver necessarily and the same goes for seats and other accessories. Factory tires are probably very heavy (12-16 ply rating) and ride like bricks. If you dont need the load capacity go down to an E or F range tire.
Just to give you an idea of some of the weights and sizes you could be working with, here are some numbers from a 2008 Mitsubish Fuso Crew Cab Chassis (no box or bed)
165" wheelbase, 254" overall length, 80" wide, 90" tall, 105" back of cab to center of rear axle, 4365 pounds on the front axle, 2070 pounds on the rear axle, 6435 curb weight (not sure if dry or wet), 5360 Front Axle Max Load and 9880 Rear Axle Max Load.
With a cabover crew cab truck, you dont have much to spare on the front end IMO, 4-5 people in the cab with some gear and you are at max. Not that the axle is going to fail, its probably got a HUGE safety margin in there and OEM are notoriously conservative about stuff like that. You also have the advantage of "easier use" carting the family around, towing a boat, etc., than heavily laden working it hard everyday, all day, etc.
