6 inch wide rims on the front won't be the problem. As I recall the stock 15 inch 6 bolt rims used on millions of GM pickups is 5.5 inches wide and they're ok. You're correct that the 16 inch wheels were 4 inches wide. You could have a problem mounting a wider tire on that 16 inch wheel...there are limits on what is safe and will work. Ask the people you'll be buying tires from. All the tire width/wheel width charts I have don't go below 5 inches for the wheel.
The back axle isn't the issue if the clearance to the fenders is okay. If not, you can have damage the sidewall of the tires and wreck the fenders at the same time. What you want with a 15 inch wheel is the tallest and thinnest tire you can get. 275/60 15 tires will have a section width of 10.8 inches (very wide) and a diameter of 28". Not good. The diameter is okay but the section width is way too wide. You'll need arms like Bluto to steer with those tires and it will cause your kingpin bushings to wear out very rapidly. Anything over about 7.5" is heading toward too wide for the front end of an AD truck.
