My suggestion is to install over the outlet tube of the muffler. My first installation was on top of the muffler with the injector right in front of the Exhaust port but I has several flameouts with the pump on at low RPM. After the move to over the exit tube the problem went away...

The stock DL Muffler with an added injector will make great smoke without buying an expensive smoke muffler... Now I do suggest that you have a bung welded to your muffler were you intend to mount your injector. The metal is very thin and won't hold a thread very well, our local machine shop charged me 10 bucks for the effort and it makes for a much better installation...

Good luck,

Ed