I would say, don't stress too badly and follow the dirctions because they're direct and as they should be, but last time I said that about a transducer I was blasted and told I didn't know what I was talking about.    

 But I've got a 620 hull and I mounted one as per directions and following "accepted" ducer mounting rules and it works great.  Keep it close to your conventional sonar if you want to overlap LSS on your 2D sonar, keep it out of the "plane" of your motor or your setback cavity.  Conventional sonar is a cone that shoots down, and LSS is like a slice that shoot out to the sides and down.  So if your motor is in the way, it'll block that side.   Mine is on the RH side, up on the step, hanging off the back of the boat to get it out of the setback angles.  Hold it up on the boat, eyeball to the left and right and make sure the shot is unobscured, and it's level.  You're mounting it on a glass boat, so don't get too worried about interference or mounting it on a block of Starboard.  Measure/mark twice, drill once!  There is also an aftermarket mount you can buy that mounts it straight to the underside of the step.  PM me if you want pics.