I was wondering what muffler is the best for backpressure and low end torque and if running a 3 inch tail pipe would be ok thanks

 

EXHAUST SYSTEM PERFORMANCE:
You have your thinking all wrong about back pressure and pipe size.
First you do not want anymore back pressure than the system design can offer.
Back pressure means exhaust gas that could not get out of the pipe and cylinders.
It takes up space in the pipe for the next exhaust cycle to push out and can contaminates the new incoming charge from the leftover exhaust in the cylinder.
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Second the pipe size is linked to back pressure.
A smaller pipe size makes the exhaust gas speed faster.
This gas actually has weight.
The speed or velocity of each "exhaust pulse" movement down the pipe developes a vacuum behind it as the cylinder's piston and cylinder pressure blow-down forces it out the exhaust port.
The vacuum or more properly the low pressure area produced, helps evacuate the cylinder and more, the pipe behind it as it exits.
As the gas travels down the pipe, it cools and slows down losing much of it's it's benifit by the time it gets to the tail pipe end.
At the tail pipe end, it suddenly loses all velocity and expands into the air allowing air to begin rushing back up the pipe to fill the low pressure area left behind it.
A note worth mentioning in a truck application is the exhaust valve closes fairly quickly (a short duration cam) such that the evacuation vacuum cannot act very much on the cylinder through an open exhaust valve event.
It's longer cam durations at higher crank speeds that benifit most from exhaust system evacuations of the cylinder when the time to do it gets a lot shorter as the RPM increases. This makes things RPM sensitive.
So you can see the relationship of back pressure and exhaust gas speed, to each other.
Actually making the pipe bigger reduces the back pressure but "loses" the speed of evacuation.
Bottom line is on a truck application, the factory pipe sizeing and muffler size has been optimized for low end torque given the already low engine torque and the truck's weight.
It's a compromise optimziation at any point because of the speed, cam durations and some other parameters that have a great effect in total, all of which you cannot improve upon here, with a couple changes.
This is the technical answer to it.
If you want to make the change for some other reasons then expect a loss of torque unless you go with the physics of it..
Even shorty header won't offer any benifit for the reasons given above.
Last point to remember is the CFM of air drawn into the engine equals the same amount of gas volume exiting the exhaust system. The only differences are the exit speed, and the temperatures involved plus the accustic effects you hear at the tail pipe end that also can have effects on total system performance in a high performance race system.
In a muffled system, the accustic effects largely get cancelled to reduce noise.
It's these differences that need to be optimized for.
It's not a case of bigger is 'always' better.
Hope you learned something about exhaust performance and why things are the way they are.
Good luck.