M900 series exhaust on M809 truck.

[MyothersanM1]

Have you thought about routing the exhaust along the underside of the right frame rail? Remove the original muffler, you won't need it with the 939 muffler in place, and tie directly to the exhaust header. I'm sure with some flexible exhaust tubing and a little imagination you would not have to cut up your cab. You wouldn't need to modify the battery boxes either.
Just a thought and observation on my end.

 

[BrushMaster Builder]

Quick Down and Dirty Exhaust Fix

The attached pics show a simple down and dirty exhasut conversion for M939 Series. We simply cut the exisiting pipe under the cab with a sawzall and bolt all the items pictured in place. Then add hanger brankets wherever we want to once the bed is installed. One thing not pictured is our turn-down. We have a 12" 90 degree turn-down that connects to the back of the muffler and puts the exhaust straight to the ground behind the tool box or fuel tank. This system will also work with aux fuel tank but you will need to put heat sleeve on your fuel lines and float level gauge wires.