We have done several conversions with Midco burners. Never had a problem.
Did a 1,000,000 btu gun on a 50 or 60 year old Weil McLain bout 10 years ago. Had one issue with it. Dirt built up on the blower wheel. Quick call to their tech support had it fixed in no time. They are good like that.
The boiler don't know what is putting the fire in it.
Clean and line the chimney,'and set it up as per Midcos instructions. You need to clock the meter to set the btu input, etc.
Just today, I looked at an oven in an auto parts store machine shop. They heat cylinder heads to straighten them. It's not firing. It has Midco economite guns on it. I'll be calling em tomorrow.
You can make it happen, you can watch it happen, or you can wonder wtf happened.
Phil Harris 2009