Yes the filter will be between the fuel petcock, under the tank and the carb fuel inlet. Don't know if you BW is gravity feed or if there is a fuel pump. Probably the later. Since you already have the new carb, it should be a snap to swap them out. You will have much better access with the seat removed. Then as you said, you can then take the old carb. down and give it a good cleaning and keep it as a spare. My DT 300i has a slight flat spot during high speed roll-on when the throttle cranked open quickly but it isn't bad enough to mess with at this time. Doesn't affect performance that much, as I don't ride it hard anyway.
Back to mid forties here today, but mild days are forecasted so hope to ride some. Take care and let me know how it all goes.
Mac