I have an L3800 - which is basically the same as a L3400 and I got a 66" box blade for mine. Mine has the R4 tires on it - which are a little wider than the AG tires. I wanted a BB that was wider than my tire footprint so I could get up close to objects with the BB. Getting a 6 ft for your L3400 should be more than adequate. Good Luck.