#3
All you need to do is remove the stock tailpiece and drill new holes at the bottom of the guitar for the new tailpiece. Easy enough mod to do and if later on you decide you don't like it then the only damage done will be 3 small holes near one of the strap buttons.
#6
I see those all the time on es-335s, and since mine is basically a clone of that, I think it'll be alright without measuring