Why are you trying to put a set of long tube headers on a 2500 with a 5.9 engine? You would be better off looking for a set of shorty headers, if you must have a set of long tube headers then find a shop that will custom build a set of headers for you.
A set of shorty headers will keep your low end torque and still give you the wow factor with all the kids when you open the hood and they see a set of headers. Long tube headers are going to make their power in the upper RPM ranges where you don't spend much time driving.
Be realistic, you have a Dodge 2500 pickup and when you load the bed or tow a trailer you are not turning high RPM's, you are in the lower RPM range where you need low end torque. Unless you are running around at 3500 plus RPM all the time you are unlikely to ever see the benefit of long tube headers.
Please don't say that the engine sounds better with long tube headers, I can change only a muffler and make a quiet engine sound like a 1960's early 1970's big block.
I had a 2007 Ram 2500 5.7 Hemi, I put a set of equal length tube shorty Edelbrock coated headers with a Borla XR-1 race muffler on it. Sounded like a big block, nice and deep exhaust note. The headers were for a Ram 1500 5.7 Hemi but you could fit them to the 2500 5.7.
It had the sound, it had the wow factor when you opened the hood but in the end with all the upgrades including a Vararam intake and an edge programmer, the end result was a lot of money spent for really nothing more than a little wow factor because it was still a heavy duty 2500 truck.