You probably won't find a Harper they haven't been made in a while. One of the main reasons to run multi carbs on an inline is to space them out along the length of the runner. The tendency is for the cylinders at the ends to starve and the center ones to be rich. It is also easy to over carb these engines especially a fairly stock one. I think a 2X2 setup where the carbs are spaced like a traditional 2X1 setup could be made to work well. You would have to set up the carbs carefully. Maybe someone else here can give some real specifications. We'd need to know what has been done to your 250 and what you want it to do for you.
