Sorry guys, just got back from Talladega. :-laf
It should be after the cat if you have one. You need to get it as far forward as possible to get in the hottest part of the exhaust. Right behind the cat will be good, it runs very hot anyways. As far as the orifice goes, braze or solder the end shut on the fitting that goes in the pipe and then drill it out with a 1/32" or 1/16" drill bit and go bigger if needed. If the smoke takes a long time to clear after you shut it off or you are running fuel out the pipe, it is too big. If you want to run diesel for the smoke, there is a auxiliary pick up fitting in the fuel tank you can hook to. You may have to drop the tank to access. PM me,If you need more info. I have run this set up on gassers in the past. I see no reason why it shouldn't work on any diesel, hot exhaust it what sets it off. If you really want to scare off tailgaters, try a flame thrower out the tail pipe!! I have made those too. Don't know if it would work on a diesel though, might have to try it.