Stack question
 

Is there any advantage to putting a single stack on a cummins instead of regular tail pipes?

no

In a single cab is the noise inside bad?

If your truck is straight piped yes its gonna be a litle more noizy but a i have a single cab to with a 4 inch straight piped 2 stack kit and its sound very good but its not too much

Okay. I don't even a tail pipe on it now. Would a stack make it louder than it is now?

With my extended cab the noise is very bearable, windows up its actually quieter than out the side, but with windows up in 5th gear its alright until 70, but after 70 it gets pretty loud. I would think it would be worse with a regular cab than an extended.

Oh and by the way, the size of the stack you run will have a large effect on how loud it is in the cab, basically the drone. A 7 inch stack will be much louder in the cab than a 5 inch. I could barely hear my 5 inch in the cab at any speed, but the 7 inch can get annoying above 70mph.

Okay. I want to get a 8" and I have a regular cab

The size of pipe will not change the sound

What will then?

What is the exhaust system you have on the truck now