Not sure of the exact setup in your 2-8-2 but its probably quite similar to the scale Mikado 4-8-2, which I have worked on. Nice engines, but the plunger puffer is a pita. Kind of lame. I tried retro-fitting an MTH smoke unit and it fit just fine, but getting it to work was another story. The problem is that the cherry switch is operated by the same arm that operates the plunger, and when you remove the smoke unit there is not enough down force on the arm for the eccentric to operate the switch. Plus you can't leave the arm in place anyway, because the smoke unit requires that space. I spent countless hours working on it with no success. Finally I gave up, gutted it and put in PS_2. Works just great now and smokes like a champ. I have two scale mikes converted to PS-2 and they are great runners. If your ERR conversion does not need a cherry switch to trigger chuffs you are good though.
Let me know if you need any more information. I also have some pictures which I can send you offline. My email is in my profile.
Rod