Have a customer that is looking at changing his 07 NBS 3500 Regular Cab Longed into a dually. From what I'm thinking all that would need replaced is the bed and the rear axle?? Is that right thinking. I'm pretty sure the frames are the same right?? And since he's got a 3500 already would it have the same amount of springs too or would those need replaced also? Hopefully I got this question in the right place

Get the spring pack with the axle they are different. The SRW trucks are only sprung for the carrying capacity of the 2 tires. You will also need 6 wheels and tires and the front hub spacers.

You should beable to get everything from junk yards. You should beable to just change out the bedside. I don't know which why would be cheaper. Worth be looking into.

I thought the axle was the same width? Isn't there just 2 widths? Standard and narrow, narrow being like a tow truck?

narrow is the C&C trucks and the "wide" is the pick-up bodied....

see if you can talk him into air ride, no springs and way better ride