You'd better get 3m's lawyers on the phone and ask them exactly what kind or financial backing they are prepared to give you in the event of a failure. Scrub that, save the cost of the call, I can tell you exactly how much money and effort 3m will put into helping you out..... zero.
Many moons ago we stuck a fancy stainless steel panel about the same size as the OP's sign using 3m tape. This was at ground level on a decorative wall.... we had the regional sales manager, the national technical manager both come by and look at the job. Following their advice we lined the recess in the wall with thick panels of acrylic which were countersunk and fixed with screws and plugs on 400mm centres.
With the wall lined we then spent a small fortune on 3m tape, cleaner and all the jazz recommended by the 3m bigwigs. The stainless was stuck in and left with timber outriggers pushing it onto the acrylic liner for 4 days and nights and then removed.
About 2 months after installation the weather turned quite chilly.... the sign belly flopped onto the car park leaving the acrylic liner in pristine condition.
3m ran for the hills at an impressive speed. Yes, they did provide technical assistance but no they weren't going to help because they didn't physically prep the acrylic & stainless nor did they apply the 3m tape. The fact that the prep and installation process including the timber props was EXACTLY what they told us to do was neither here nor there... 3m employed hands did not touch the prep or installation so that was that.
This kind of "we didn't prep or apply our product" get out clause is standard across the entire adhesive industry. So are disclaimers which state that heavy loads should not be suspended without additional mechanical fixings.
If you want to bet your entire farm on the willingness of an adhesive supplier to "do the right thing" then you're very brave or very stupid.
Mechanical fixings are cheap, quick to install and provide that added "belt & brace" factor.... something which helps you sleep at night.
thats just how us crazy canucks roll 
:Canada:
